New to Forum/ Seeking help regarding a bad smell in our home

[OSUCarrieA]

Our home has a bad smell only in the winter months when the air conditioning is not running. We can smell it coming from several vents and in the basement where the heater and hot water tank are located. The smell is similar to rotten eggs, sewage and we had someone mention sulfur. We cannot seem to pinpoint what triggers the odor, except that we do NOT smell it when the A/C is on. It seems to come and go both when the heater is running and when it is turned off.
We moved in a couple of years ago and have smelled the odor both winters. We cleaned the de-humidifier (which was disgusting – we thought was the source) and have turned it off completely, but the odor still returned. We have an aerobic septic system which has been checked out; we’ve asked a plumber about the smell, who didn’t have an answer. He did suggest we fill the traps with water, but that has not made a difference.
From researching online, we have some theories – we’d like to have the ductwork cleaned, but are discouraged after reading that other home owners have spent thousands cleaning their ductwork and without eliminating the odor. My husband has a few theories, including replacing the hot water tank. He’s also tempted to tear down a wall to see if there is a possible leak; however this would not explain why we do not smell the odor in the summer months.
We appreciate any feedback, or suggestions on what professional might be able to help find the source. Thank you!

[driz]

I once had a tenant with a similar problem. It turned out to be a dead neighbors cat that had crawled into a tiny place gotten it's collar hooked and died there. That was in a mobile home and though the cat was not in a duct itself but between the floor and the insulation under it the smell seemed to always be coming from the ducts themselves. Poor cat.... Anyways it lasted for nearly a year and tapered off and went away slowly over time but it did have that sulphery moldy stink you describe. In your case though I can't see it lasting over the course of two full winters.
Hey, the last owner wasn't creepy was he? The cop in me sort of wonders if the prior owner doesn't have a missing wife, ex wife or a penchant for entertaining runaways ect. He wasn't by chance related to John W Gacey was he? You never do know about people.

[dkpbxman]

Field mouse

One time a small little field mouse died in my attic. The attic is unfinished so what you see are lengths of insulation in each space between the floor joists.

The mouse was sandwiched between the insulation(under it) and the first floor drywall ceiling when he died.

In that same joist bay, very close to where the mouse was, a hole had been cut in the drywall, for a pipe, coming up out of the hallway closet directly beneath it and running through the attic.

I noticed that my clothes from that closet smelled awful. I took everything out, cleaned everything twice and the smell was still there. Then I noticed that hole, checked the attic and found Mickey, steadily decomposing.

It's amazing that such a strong smell could come out of something so small.