Out of no where, a very foul odor has been coming from just one cool air return vents when the heat is on or off. It seems to be spreading throughout the house. What is it and who do we call for service?

Sounds like a dead rat,, does it smell like one

Never smelled a dead rat before and have not seen any trace of rodents in our house--it certainly smells like a dead 'something'... it is now coming from a second cool air return vent in proximity to the first site of foul odor.