It's been suggested that it might be a crack in the vent pipe, which I guess makes sense. I've been on the roof and the pipe doesn't seem clogged (at least where I can see it), and the trap in the kitchen sink is newly installed.
My questions: 1. Is there any way to pinpoint where exactly a leak is (if one exists)? The AC unit isn't especially complex, and the ducts aren't that extensive; I can see almost all of the duct length by standing on a chair and shining a flashlight into the vents. I don't see any possible leak though. The air conditioning unit on the roof for my apt isn't close enough to the vent pipe to suck in any smells either.
2. Should I call a plumber or an AC repair person? Presumably a plumber would be needed to actually repair any pipes, but I would think an AC person would be better able to find the leak.
3. Is there a way to search for leaks without taking apart walls?
4. Is there any possible other explanation for the smell?
If at all possible, please post a response here, my email account is inaccessible at the moment.
Thanks for any help anyone can provide.