Noise & smell with fan/ac

Sorry to say, your correct. You need to have the HVAC box removed immediately to remove dead whatever before you have to sell the vehicle.

I've seen mice and rodents build nests in the air filter housing (air intake into the engine) and air intake ducts for the heating and ventilation ducts. After debris is removed from the ducting or fan motor spraying Lysol disinfectant in the air ducts will help kill germs and bacteria and clear the bad odor.