[Congressman Billybob]

May be dead mice in the ductwork. Human or animal, death has a certain strong sickly-sweet smell.

John / Billybob

[Ben Ficklin]

I suspect the mice also.

It doesn't have to be a dead one either. A build-up of droppings can be obnoxious smelling when heated. Years ago the mice built a nest in my kitchen range and the smell was awful.