how long ago did you get it serviced? your supply vents might be dirty
and musty, and its releasing oders as those vents gets hot, and
disaapears when it reaches centain temps.

and if not the supply vents check the exhaust vents or flue, make sure
its clear of any combustibles close to it, should be away 6" or more.
and no cracks, breakage or holes it in. if none time to check the
inside and service it. i suggest a professional.

is there a damper? what kind of damper do you have in it? motorized or
mechanical? that might be broken, if so youre smelling carbon
monoxide, very deadly!

seen some vents that are exhausting to the attic, or the flue has no
rain cap, improper flue slopes and a handfull more.

i think service is around $50...........