For about the last year, we've been wrestling with a horrible sewer odor downstairs that comes and goes. It seems to come from the crawlspace access, yet I've had a plumber and another technician go down there and not see anything out of the ordinary. The smell can be like a musty wet mildew smell or like the subject tag says "sewer". We do have a HE washer, but we've cleaned this with cleaning tabs bought at the Home Depot recently. The smell will change with the weather as well. Whenever there is a change from hot to cold or vice versa, it gets worse.

Bob