It could the septic system. It might not have been installed correctly, and some part of it is freezing and backing things up.

Get a new plumber. Methane is odorless and colorless. Natural gas, which is mostly methane, has to have an odorant added to it for precisely that reason.

I recall that when we were building we had an unpleasant smell coming from the separate carriage house area. It had a laundry area with a drain in case of flooding. We tracked the smell to the drain and my husband suspected a mouse had found it's way down the drain some time before the small grate drain cover was installed. He explored that idea and found a small dead mouse down the drain. If I remember correctly there was water in that drain and the water and decomposing mouse gave off the most obnoxious smell. The mouse must have found it's way in when the garage door was left open. We live in the country. Your problem could be big but it could just as easily be something unexpected like the one we had.

Do you also smell it on rainy days? I don't know about basement, but my attic (where Studor valve is located) and top floor smell whenever there's a drop in atmospheric pressure. We also had really bad smell in powder room right after moving in - found we didn't have a good seal in the toilet flange (though no water leaking, had gas) b/c of grout under one edge from when guy put in tile causing gap on other side. DH pulled out the toilet and flange, scraped away the grout and reset the toilet, it's been fine since.

Check and see if there is an unused outlet capped off in the basement, the smell could be coming from there.

Thanks for all of the info and suggestions. Creek side, I agree that it is probably the septic system. And we are now fairly sure it is caused by the roof sewer vents freezing up. Most often the smell has gone away after my husband broke through a thin layer of ice on one of the vents. The reason we thought it might be something else is because our plumber told us that a thin layer of ice usually doesn't cause sewer gas to back up, and he also said he didn't think it smelled like sewer gas. But what other explanation could there be? Why else would the smell come and go so often, if it isn't the sewer vent freezing and thawing?

Last night confirmed the fact, at least to us, that the smell is coming from the freezing vents. It was seriously stinky last night and much more of a rotten egg sulfur smell. So DH went up to the roof and knocked ice off of one of our vents. This morning, the smell is pretty much gone.

So if the smell is the result of ice forming on the vent, then does anyone have an idea of why our one (not both) roof vent is freezing over at least 3 times a week? And how can we prevent it so that DH isn't climbing on the roof so often?

Why have I never heard of people having to unplug their sewer vents so often? It seems that something must be seriously wrong for that to keep happening.

You might run this by the plumbing forum and ask for LazyPup. He's a master plumber and offers great advise/troubleshooting.

Thanks, sierra. I posted the question over at plumbing. Hopefully someone can give me some suggestion or idea of what to try to get rid of this problem!

I'm wonderting if your plumbing is vented correctly? Our neighbors have that same problem of the vent freezing, they said something about the size of pipe being wrong going thru the roof. We also have problems with odors. We know what the cause is, but haven't figured out how to fix it. None of our sink drains were vented, also the tub/ shower was not vented. I smell this all the time, my nose is sensitive, hubby never can smell it. I especially smell it every time the weather changes or the temps. I hate it and want it fixed. I'm afraid walls will have to be tore out, I don't care, it is convincing my hubby to do it or have it done. He thinks because things work that all is well. i wish he could smell this. Maybe this will give you some ideas. It's no fun smelling this, I know. I am tired of burning candles, spraying air freshners, running the exhaust fan.

Ours (when toilet wasn't seated right) was worse when we ran the exhaust fan - it's like it pulled the fumes right up into the room!

We still have the problem with the Studor valve, it's for the powder room in the front of the house but builder didn't put separate stack in. They left part of the attic floor between joists open for plumber to run over and join the main stack but he put in the studor instead. I told him about the odor (and called for estimate on iron filter so it would be worth his while to come out again), but he never came back. We're going to do something about this while we have the attic floor pulled up again to move lights when we finish the master bath.

I'm another one who smells everything when DH can't smell *anything*. This drives me crazy on rainy days!