In the spirit of being genuinely very far off topic and drawing on this community's vast knowledge base, I have a question for any of you who have plumbing experience or septic tank experience.

I live in a single-story ranch-style house built around 1960, where we have been for about five years. The home is "in the country" -- or at least outside city limits, meaning there are no city sanitation services. We have septic tanks, which I assume are original to the home. They are concrete and have passed a couple of inspections, so I think they're fine. There are two main tanks in series which service the toilets, sinks, showers, and so forth. The kitchen and laundry have their own grease trap and tank with a separate set of field lines. Everything seems to work as it should with stuff going to the tanks. No leaks, nothing backs up, everything goes down, all good.

Our problem isn't with solids or liquids, but with gases. Periodically (but not constantly at all), our master bathroom gets quite the unpleasant whiff of sewage. It's bad, really bad, and I will spare you further details. It doesn't seem to matter whether the tanks have been pumped recently or not, and it does not seem to depend on the time of day, the season, or anything like that. The only trend I have been able to sort out is that it's most likely to stink when water is running in the master bathroom (shower or sinks, but especially the sinks) at the same time the central air conditioning system is running. Flushing the toilet does not seem to cause it, and no other rooms / bathrooms have this issue.

My only guess is that we have some kind of a venting problem, and the septic system is somehow venting into my bathroom. The sinks both have standard p-traps on them, so I don't think it's as obvious as sewer gas coming back up the pipes -- but I don't know how else it would be getting in the house. Also, I don't hear bubbling or gurgling. I'm not sure whether it's relevant, but the master bath is the furthest point of the house from the septic tanks.

What sage advice can you guys share? Is there some obvious DIY repair (that isn't obvious to me) to deal with this, or is it time to bust out the yellow pages and call a plumber?

Thanks!
Cheers, John