I got dead rodent in return duct, at least I hope I will find it there. My returns on bottom and supply on top. Strong smell from return in one bedroom. Attached is pictures how this duct look from a basement. One end is next to window (where it goes up to the room). Another picture is where sheet of metal meets another one. It's about 8ft long.
I need to take that section out, I believe this is where rodent will be.