The car sat for a week while I was away. It was completely closed so no water got in. When I returned home, there was a terrible smell coming from the vents; it smelled like a barn. When I turned on the vent, AC or heat, there is a lot more vibration than there was before. What is going on?
Sounds like you have a camper in the squirrel cage.
The squirrel cage is the air pickup for the fan motor.
Probably a mouse.
You may have to dismantle some of the ductwork or at least the cover of the fan motor to see.