Why don't you temporarily seal all the old ducts to see if the odor goes away?
Ron

What would be the best way to seal the old ducts?

I have been reading some of the other questions posted on the site. I believe the odor may be coming from the dirt floor that is exposed under our house. It is also a very tight squeeze under the house. Where would I look for someone to do work on my dirt crawl space? (remove dirt, lay barrier down, etc.) I would do it my self, but I don't have any skills. I can't even cook a hamburger or change a tire. My husband is as unskilled as I am. I have the drive and the willingness to do projects, but no know how.
Tamara

I wonder if it could be the mice? I have never noticed dirt floors causing an odor through the ducts before; it seems animals might be more likely. I used to live in a rented house that had that problem, and until it was resolved I would boil oranges on the stove in a big pot with plenty of cinnamon and ground cloves added. That helped a lot!

Tamara- I don't have an answer but thought I'd chime in and tell you that our house suffers from the same odd smell plague. We, too, have old vents in the floor that weren't removed and a tight, exposed dirt crawl space. I hadn't even thought of that as a possible cause. I look forward to reading everyone's advice to you.

Folks, I suspect that Tamara is either long gone or has solved her problem by now. You're posting in a 2007 thread that some slimy spammer resurrected. I hate to see you waste your time, though it's interesting to read!