It could be one of two things, a sewer vent pipe made have come apart in the attic if it's above you or because of air conditioning, the drip pan inside the air handler can get awful nasty with mold, mildew and plain crud that can smell. The ac coils usually can get mold on them but since it's now only heat mode, the drip pan and coils should have dried out. Cleaning the coils with coil cleaner and disinfectant poured into the drip pan and washed down with water, the condensate line cleaned in the process maybe what's needed. This is standard maintenance for air conditioning. Those that live in the snow belt usually cannot comprehend that AC units needed maintenance.
Where is the furnace located And air handler located. Is there attic space above you?