I have a two story townhome in Orlando FL. The drain pipes from an upstairs bathroom go down through the walls of my pantry downstairs in my kitchen. After a long rain (1-3 hrs.) my pantry began to smell like rotten eggs/sulpher. It always subsides after a couple of days wth no rain but returns quickly once it rains again. The AC lines also go through the same penetration in the slab. I have removed the sheetrock and foamed the area where these pipes go through the slab but the smell still comes back. Any suggestions?

Your vent stack may be plugging up and 3 hrs of rain is just enough to temporarily block it. If you can safely reach the roof stack I'd rod from there.

Thanks, one more question. If it is the vent stack why would it stink at the bottom of the house in the pantry and not the bathroom directly above it which would be closer to the vent stack?