I have been in both the crawl space and the attic to see if that is where the smell is coming from. It is not. The smell resembles something rotting, i.e. food/cabbage. The smell is only coming from the register in my son's room. The duct is run in the attic and the register vents are in the ceiling. I have disconnected the section of duct, about 10 feet long, inserted a large flash light and then removed the register cover at the other end and looked into the duct with a mirror. I was expecting to find a rodent dead, but it was clean as can be. The rest of the house is free of the smell, but it is clearly coming from around the ceiling in that room.
What could this be, and where is it coming from. This is driving us crazy. HELP!