If you are lucky, there is a male ginkgo tree or a cobra lily blooming near your work room. Many beautiful ornamentals smell like urine and worse. However, methinks you smell a rat. Rat urine is very smelly and a dead rat goes through many unbelievably stinky stages of decomposition. To see if there's something in your duct, cover all the openings with plastic or foil, wait a day, and then uncover one end and give it the sniff test. Sorry, but I don't know how you might clean out a duct.

Thanks! I will give it a try. The only tree close out outside is a crabapple tree. It's annoying always getting hit with falling apples but it doesn't stink. I have seen evidence of mice in our garage. However I have never seen evidence in the shop or the house who know though mice can get anywhere. :(

Do you have a crawl space under your house? You could check there and also in the attic.

No crawlspace or attic over the shop portion of the house. We are taking off vent covers etc. this evening see if we can see anything.

Thanks a bunch for the suggestions.

I know someone who used Dream Clean and they were very dissatisfied. Don't call them. They shouldn't be advertising here anyway so I guess that shows their work ethic by advertising where it's not appropriate.

I would try a duct cleaning company if you think it's in the duct work. Safety King is the big one out by me, not sure if they're national or not. They would probably know the best way to go about actually cleaning the ducts.