Help diagnose a rear end nose

[soaringDude]

Hi folks,

I am getting closer and closer to finish line and I have to say it has been super exciting to have a car that I can actually start and go for a spin in after years of it being a hung of metal!

However I noticed when I am cruising (in neutral or clutch pressed) there is a clanking sound from the rear end. My first thought was something is binding to the moving parts. Put the car on jack and looked around, rotated wheels and drive shaft and no binding or noise. Clearly it is a rotational thing and you can hear it slow down as the car slows down. Any suggestions? The car is a MK4 with the older IRRS setup and the rear end was rebuilt with completely new gears by a local shop (4 years ago!) and it has 10 miles or so on it, almost all of it below 25mph.

Any ideas, suggestions? You can hear it in this clip:


https://youtu.be/_KUUwymN4ZQ

[Mastertech5]

You are going to need to run it on jack stands so you can isolate the noise. That's not a familiar noise to me as a mechanic for 40 years. There are a lot of noises that I have cataloged in my head over the years but that one doesn't ring a bell.

[NAZ]

Are you saying it only happens when the clutch is disengaged or in neutral or is that the only way you can hear it above the engine noise? I'd get a helper and run it with the tires safely off the ground and the chassis supported (jack stands or a lift) as it's very difficult to tell where some noises are coming from when driving or even setting in the driver's seat.

I've heard brakes make a similar noise. If you drag the brake does the noise change?

[SJDave]

Well that sucks!!! You were doing so well, hate when these unexpected problems show up.

I'm free tomorrow afternoon if you would like some help diagnosing the noise, 2:00?
Dave

[dhuff]

i have heard a similar noise. it turned out to be a piece of metal stuck between ring gear teeth and the noise occurred when a pinion tooth hit it.

Could also possible be a wheel stud that was not pulled all the way in and is hitting a brake related part.

[soaringDude]

Putting her on jack stands and running is the next plan but I don't know if the wheels will have enough inertia to turn after you engage the clutch to hear anything. But at least it will confirm that I am not getting the noise when under power. Very unusual sound indeed...

[soaringDude]

I think it only happens when there is no load (no power coming through driveshaft) with either being in neutral or clutch pressed. Not 100% sure, but even in very low RPM under load I couldn't hear anything otherwise it is pretty clear and obvious.

I am guessing it is not the stud unless it only happens with the shocks under load. On jack I rotated everything and I heard no noise. Running it on jack will definitely help isolate it a bit better. Also I will remove the trans tunnel cover, it should help too.


Are you saying it only happens when the clutch is disengaged or in neutral or is that the only way you can hear it above the engine noise? I'd get a helper and run it with the tires safely off the ground and the chassis supported (jack stands or a lift) as it's very difficult to tell where some noises are coming from when driving or even setting in the driver's seat.

I've heard brakes make a similar noise. If you drag the brake does the noise change?

[soaringDude]

I just noticed this. Great suggestion, I didn't try that. Will report back...


If you drag the brake does the noise change?

[Ian G]

Did the person who assembled the rear end torque the ring gear to carrier bolts properly? I had the same type of noise in my roadster. Under load it was ok, but coasting sounded like yours. I found half the ring gear bolts were only loose. The rest had fallen out. Caused some major damage. That was the only part of the build I subletted to another mechanic (Ford guy). I did it the second time and it's been dead quiet ever since.

[BadAsp427]

Just a guess, but I had nearly the exact sound. The bolts that attache the drive shaft flange to the rear end flange may be too long. Sure you have them torqued, but they could be bottomed out and are not actually pulling the two flanges together. Here is my first drive video (https://thefactoryfiveforum.com/showthread.php?29472-Carl-s-20th-Anniversary-8690-10-000-miles-With-Jeff-Kleiner&p=345707&viewfull=1#post345707) Part of the sound is my GoPro rattling around inside it's case.

[soaringDude]

The more I play around with it, the more I am worried that it is a assembly problem with the rear end. Sadly my story is the same, this is the only part I got a pro to do. I will call them Monday morning. Will do a bit more testing on the jacks tomorrow too.

BTW braking while coasting does not stop the sound and I am pretty sure at this point that under power, there is no noise or certainly not as noticeable as when coasting. Unfortunately that assembly job goes back almost 5 years when I started the project ��


Did the person who assembled the rear end torque the ring gear to carrier bolts properly? I had the same type of noise in my roadster. Under load it was ok, but coasting sounded like yours. I found half the ring gear bolts were only loose. The rest had fallen out. Caused some major damage. That was the only part of the build I subletted to another mechanic (Ford guy). I did it the second time and it's been dead quiet ever since.

[soaringDude]

I checked all the bolts on the flange last night and they were all pretty tight and solid. I will check again when I put her up tonight.


Just a guess, but I had nearly the exact sound. The bolts that attache the drive shaft flange to the rear end flange may be too long. Sure you have them torqued, but they could be bottomed out and are not actually pulling the two flanges together. Here is my first drive video (https://thefactoryfiveforum.com/showthread.php?29472-Carl-s-20th-Anniversary-8690-10-000-miles-With-Jeff-Kleiner&p=345707&viewfull=1#post345707) Part of the sound is my GoPro rattling around inside it's case.

[Gordon Levy]

I have heard that noise on a live axle car and it was the u joints bottoming on the flange. Generally not something you would see on an IRS but with the rhythm on the noise it makes sense.

[Andrew Davis]

Have similar hardware, found some obscure instructions about spacers under trans mount. Sure enough, once loaded with car weight the drive shaft was impinging on frame. The shims (washers) cured my issue. Hope it helps.AD

[soaringDude]

Any more you can share? I am not sure I follow. I set the driveshaft angle with the spacers under the trans mount. That angle doesn't change with load or weight. But I will look for any sign of things hitting in that area. Definitely like to hear more as I am guessing I didn't understand your suggestion.



Have similar hardware, found some obscure instructions about spacers under trans mount. Sure enough, once loaded with car weight the drive shaft was impinging on frame. The shims (washers) cured my issue. Hope it helps.AD

[soaringDude]

The angle is pretty shallow and given the reared is fixed, not sure how that be the case. I will take the driveshaft loop off and measure the angle again to confirm. It has been on my list of things to do for a while anyways to verify it.


I have heard that noise on a live axle car and it was the u joints bottoming on the flange. Generally not something you would see on an IRS but with the rhythm on the noise it makes sense.

[rich grsc]

Not a drive shaft issue, unless it's a failed U-joint. On an IRS car you hardly even need to think about U-joint alignment, the shaft is almost straight, compare it to a solid axle car where the axle is constantly moving up and down.
Have you checked wheel clearance, a balance weight?

[Norm B]

It seems to be too low of frequency to be drive shaft related. That leaves internal differential, cv joints, wheel bearings/hubs, wheels and brakes. I have heard a bent brake rotor cause a similar noise but that wasn't dependant on power on or off. An incorrectly assembled cv joint can do the same but that usually causes a vibration under mod acceleration.

The next time you have the wheels off the ground try turning them backwards and listen very carefully. If you could find a way to keep the suspension at ride height that would be best.

Good Luck

Norm

[soaringDude]

I didn't notice any other abnormal vibration but then again I didn't push her too much. I only have 11-12 miles on her. I turned the wheels forward, reverse,... I spent a good hour or two under the car last night looking at every bolt, every part that moves to make sure its nothing stupid or obvious. Dave is coming over in an hour and hopefully with him I can run the car on jacks and listen closer and see if we can reproduce the sound on jacks to better isolate it. I guess I have to cancel my alignment appointment tomorrow. Don't want to drive 14 miles without knowing what this is.



It seems to be too low of frequency to be drive shaft related. That leaves internal differential, cv joints, wheel bearings/hubs, wheels and brakes. I have heard a bent brake rotor cause a similar noise but that wasn't dependant on power on or off. An incorrectly assembled cv joint can do the same but that usually causes a vibration under mod acceleration.

The next time you have the wheels off the ground try turning them backwards and listen very carefully. If you could find a way to keep the suspension at ride height that would be best.

Good Luck

Norm

[soaringDude]

Yeah, I am pretty sure it is not the driveshaft either. First the sound does come from further back than that and second, I got a local shop to balance it a couple of months ago. If there was major failure of the u-joint, they would have noticed it as well and I had specifically asked them to inspect the u-joint and its bearings.


Not a drive shaft issue, unless it's a failed U-joint. On an IRS car you hardly even need to think about U-joint alignment, the shaft is almost straight, compare it to a solid axle car where the axle is constantly moving up and down.
Have you checked wheel clearance, a balance weight?

[Gordon Levy]

I still think it is drive shaft related by the way the shifter bounces at ever sound. Just throwing it out there, did you remove the tailshft dust insert for the trans before you installed the driveshaft?
The other thing to check offhand would be when it's up in the air grab the pinion yoke and give a good shaking and see if it moves. Double check the pinion nut torque.

[soaringDude]

Thanks to Dave for coming over and helping with the diagnostics. Here is what we learned. First of all he went over everything to make sure it is nothing stupid that I messed up (totally possible!). We could reproduce it but much quieter on jack stands by putting her in 2nd gear, let the wheels get some speed and engaging the clutch. Then we managed to reproduce it by turning the wheels. Turns out if you just turn on side wheel forward, you hear the noise exactly every two revolution. Here is the kicker, if you turn both wheels, the frequency increases to closer to 1 clank per revolution but hard to truly believe that number as it was hard to spin both wheels at the same speed. We heard no noise from the transmission or driveshaft side of things. And given it is a once per two revolution, I am guessing it is not the CV joints and the axels.

Dave also saw some online reports of TrueTrac differential making this noise when coasting. I feel safe enough to go for my alignment appointment in the morning and then talk to the shop that built the rear end for me. Thoughts? Suggestions?

I also wanted to say again - I am so grateful for the community and these forums. Amazing how everyone pitches in to help when one of us needs it. Appreciate all of you!

[soaringDude]

A quick update. Took a sample of the oil from the center section. While it is pretty cloudy and dark, there was no metal shaving there. Also finally talked to the one of the guys from the shop and he diagnosed it as a nick or burr on a gear tooth that under acceleration does not cause a problem but when coasting creates that sound. His suggestions was to do a few burn outs in reverse! That will have to wait a bit while I break in the engine and the rest of the drivetrain but at least I am comfortable driving her and finishing off various inspections and most importantly alignment.

Thanks for all the help. Once I resolve the problem I will post an update for others. Back to the build!

[cob427sc]

Burnouts in reverse? first I ever heard of that to solve a rear end gear problem. Let us know if it works.

[NAZ]

We need an emoji for shaking your head in disbelief. Fixing gear damage by doing burnouts in reverse -- that just raised the bar. Like fixing a leak by draining the oil.

[Jacob McCrea]

For what it's worth: A few years ago I built a spare 8.8 IRS differential (T-bird style, not the new style) with a Detroit Truetrac. I seem to remember some shaft, pin, or something to that effect, and there was a question about whether that component was required for an IRS application, as opposed to installing the Truetrac in a solid axle. I seem to remember that the piece needed to be installed, but I forget how it was retained in the unit. I'll check my instructions, and the diff if necessary, to see what the deal was with that part. Maybe it's banging around in there, although you'd think you'd get some serious metal shavings if so.

Edit: This is the part I'm talking about:

https://www.summitracing.com/parts/dtl-52733?seid=srese1&gclid=CjwKCAjw97P5BRBQEiwAGflV6cNyznzqhC9-Grse5QwoH2LNnQ4mwG0xFcdDKIlE6fhmo9Q-AubClxoCWpkQAvD_BwE

[soaringDude]

I really have a hard time believing it would work but his explenation sort of made sense. If you have a bur on the back side of the gears (that does not usually sees force), going under power in reverse, the back side of the gears is the side that transfers the force.

My current plan is to put on a few hundred miles on it and see what happens. If it goes away, perfect. If its doesn't, I will pull it out as a winter project and try to figure out what is causing this.


We need an emoji for shaking your head in disbelief. Fixing gear damage by doing burnouts in reverse -- that just raised the bar. Like fixing a leak by draining the oil.

[soaringDude]

Very interesting. To be honest I am not sure I even know what that part is, but I make a note of this. Thanks for sharing.


For what it's worth: A few years ago I built a spare 8.8 IRS differential (T-bird style, not the new style) with a Detroit Truetrac. I seem to remember some shaft, pin, or something to that effect, and there was a question about whether that component was required for an IRS application, as opposed to installing the Truetrac in a solid axle. I seem to remember that the piece needed to be installed, but I forget how it was retained in the unit. I'll check my instructions, and the diff if necessary, to see what the deal was with that part. Maybe it's banging around in there, although you'd think you'd get some serious metal shavings if so.

Edit: This is the part I'm talking about:

https://www.summitracing.com/parts/dtl-52733?seid=srese1&gclid=CjwKCAjw97P5BRBQEiwAGflV6cNyznzqhC9-Grse5QwoH2LNnQ4mwG0xFcdDKIlE6fhmo9Q-AubClxoCWpkQAvD_BwE

[NAZ]

Dude, here’s something to consider. That noise you’re experiencing is very likely causing damage or excessive component wear. The audio quality of my laptop is not sound studio quality but to me it sounds like something rubbing similar to the sound of a brake rotor with excessive TIR. It is not likely to go away with use – mechanical problems don’t heal themselves. If you have narrowed the problem down to the rear end then you need to pull the cover and check the internals. On the IRS I believe you will need to pull the unit.


If you bought a new Mustang and it had this problem, you’d expect the dealer to repair it under warranty, not tell you to drive it and maybe it will go away. For a mechanic to tell you to do some burnouts in reverse to fix a chipped tooth or burr on a precision ground and hardened gear set is beyond stupid. Be smart – cross this Bozo off your list of repair shops.

Hypoid gears are designed to carry a heavy load in one direction only. Burnouts are an example of a heavy load. The buttressed or asymmetrical gear teeth are not designed to carry high loads in the opposite direction. You can see how the gear teeth in the attached photo are tapered on the back side and less taper on the drive side.
133294

[Mastertech5]

I totally agree with Naz! Get yourself a mechanics stethoscope to isolate where the noise is coming from, just don't touch it to any moving parts or your ears will pay the price! If you've found the problem by now, never mind. Update please when you know. I'm very curious about this.

[soaringDude]

A quick update on this. I dropped the rear end and opened up the pumpkin and inspected everything and found no issues. Our best guess was a nicked gear tooth but we found none and as expected when I put it back on and drove, the problem was still there. At this point multiple people have looked at this and we have had a tear down of the rear end and found nothing out of spec. I read some reports of some eaton differentials having this problem but three isn't much you can do about it if that's the problem. In any case, I will put some miles on it and see what happens. Very strange issue...

[luvaz]

To the OP,

Lots of good advice here, especially from Gordon, IMHO.

FWIW, I live down a mile of washboard, unmaintained dirt road in the desert. It doesn't take very long for a daily driver to start making suspension-related noises in these conditions. With that, I have found mechanic's stethoscopes to be very useful, but they also have their limitations. To help isolate and identify the source of dynamic road-noise, such as yours, I have found chassis ears to be a great tool designed specifically for this purpose! While they are not inexpensive, they do the job. There are wireless types and wired. I have a wired set that I bought used, online a few years ago. Look around, ask around, see if anyone nearby has a set you can use, or dive in and buy a set. I've also found that having a 2 post lift (I've also had a 4 post, but that's another story) helps to troubleshoot undercarriage issues immensely.

Here's a link to an episode of Powernation's Truck Tech TV show, where they used a set of chassis ears to successfully troubleshoot an intermittent noise on a staff members Jeep:

https://www.powernationtv.com/episode/TT2015-06/sonic-suspension


I hope that helps, keep us posted!

Will

[AC Bill]

We need an emoji for shaking your head in disbelief. Fixing gear damage by doing burnouts in reverse -- that just raised the bar. Like fixing a leak by draining the oil.

And that was suggested by the shop that did the rebuild..:confused: