(I'll preface this by saying I'm not an experienced DIYer, especially with plumbing, but this seemed to be the best place to post this question. Hoping someone can help!)
I've had a sporadic smell, like something rotten, from under my bathroom sink for several years. I've cleaned out the p-trap several times, and it hasn't made a difference. I have to use baking soda & vinegar and/or boiling water to clean it out (after that, there's usually still a little bit of an odor if you stick your head under the sink, but it doesn't permeate the whole room).
I had a plumber take a look, and his best guess was that I've got a crack in the septic vent pipe behind the toilet, and the smell wafts over and through the hole around the pipes under the sink. He suggested, short of taking down the entire wall and replacing the pipe, that I stuff the hole with cloth to block the smell (which I did, but this hasn't been effective).
The weirdest part of this is the trigger. When only a little water is run into the sink, the smell starts within an hour. If we flush the sink out every time we use it, the smell generally doesn't build up. This is why I'm not convinced that the septic vent pipe is the issue - if it were a crack, it would presumably be a problem all the time.
A few notes about setup: this bathroom is upstairs, with a second bathroom directly below it (which does not have a smell issue). Second bathroom is used regularly, except for the shower. We have a septic tank, which has been emptied during our time here and which did not affect the smell. Town water, so it's not something in our well. Running the bathroom exhaust fan increases the smell, but that makes sense, since it pulls the air up through the room. Opening the window is very effective, but not so practical year-round.
So my main question is this: why does a little bit of water going down the drain cause a funky smell, but lots of water does not? How do we resolve this? Does this sound like the sink is the problem, or the septic vent, or something else entirely?