We had our heat and air ducts moved to overhead because the ones in the floor were rusted out. This was about six years ago. Starting about 18 months to 2 years ago we have noticed a bad odor (smells to us like dead animal only not that strong) coming from only one overhead vent. This happens intermittently. It may happen twice in one month or may not happen for 3 months. I don't think it is a dead animal because it only lasts for about 2 hours and then it is gone. The vent is close to our gas hot water tank (there is a floor drain also located there). We have poured water into the drain weekly but that has not stopped the problem.

It could be a desiccated dead animal that smells now only when wet. Does this happen after a rain?
I'm getting rid of our air vents entirely because of this. Our house has dead mice in ceilings, walls, floor, appliances, under cabinets, you name it.

Have the ducts and coil cleaned. If the floor drain smells like the air it may be sewer gases. Call a plumber or poor bleach in there.

No smell in drain. Plumbers have checked house for leaks. One found at other end of house and pipe was fixed. Smell returned two days ago for about 1-2 hours then was gone. Returned last night for 1/2 hour and was gone again. WE NEED HELP.