About a month ago, I noticed a very foul odor in my upstairs bathroom and I was almost for certain that it was coming from there... but to my surprise, it was coming from the downstairs bathroom (the 2 bathrooms are stacked one on top of the other).. I found out that its coming from the downstairs bathroom because I went to the closet in the bedroom adjacent to the pipes... I unscrewed the wall piece to see if there was a leak behind the shower... VIOLA... I found the source of the smell and the gnats... but no leaks!? I bought some "Instant Power" main line cleaner, and it worked. For about 3 days I had no smell. And then it suddenly came back. So I thought maybe the toilet needs the wax seal replaced cause maybe the toilet water is leaking around the flange... Wrong again!! The p-traps have not dried out and I have tried everything I know to do so far. Not even the plumber can figure it out. HELP!

THANKS :D

SHAY

P.S the house is 36 years old and most of the pipes are cast iron.

Any chance an animal died in the walls? (squirrel, chipmunk, mouse, rat etc). They will cause a bad smell for a few weeks - and you typically notice the smell in your bathroom because of the holes into the wall cavities for plumbing.