Need suggestions please! We are getting sewer smell in our basement when it gets cold or when lots of rain. Had a plumber come out last year, it had warmed up so no smell, but he checked the vents on top of house and said there is no blockage. Had the septic pumped a month before he came out thinking it was that. We do have floor drains in basement, but they go to drain outside our home, where gutters drain to (underground pipes that drain to it). The odor is at the point where sewer pipe exits the basement wall. I do not see any dampness on the wall, and sometimes , not always, it is much stronger when someone showers.

Is there a cleanout inside where the pipe exits the foundation ? If so maybe the plug needs to be removed , doped and reinstalled.

or maybe need sealer around the pipe where it goes thru the foundation??
if you can get up on the roof, you can dump a bottle of peppermint oil down the roof vent followed by somw warm water. that will create a smell that will help get you closer to solve the mystery..