I have a Armstrong Air Tech 91 electric/gas propane furnace and it has attached AC. When it starts up in the last 3 to 4 weeks it has a flatulence smell I guess is the best way to describe it. It lingers for a bit after it shuts off then disappears. I don't hink the system is dirty but I could be wrong but it is a good system and not that old and we had it cleaned about 4 years ago. It has never had this smell and it isn't the burning synging smell you sometimes get at the beginning of the season when you first use the furnace. Oh we are only using the Electric part as propane has gotten more expensive than the electric. Could I have a dead animal i.e... mouse, rat etc in the duct and how do I get one out if I do. Any Ideas I have my house listed to sell and it is quite a good home but this is not going to help sell it.

((flatulence smell))

Could be the smell of propane gas. I would have it checked NOW.

It's not we did have it checked and it wasn't leaking.

Is the heat exchanger leaking in the furnace? If not do so.

Dead animal is possible.
Open sewer line close to air intake is possible.
Nasty A/C coils in furnace create a dirty sock syndrome.