Heater / airconditioner smells while it's on already have it cleaned? ...
Answers: Has the filter been changed? Are you on a crawl space? Are in that things around your unit or return nouns vents that could be cause the smell? What is the smell? I agree more info needed.
Something could be in one of the ducts or possibly the motor is making an odor. It's liberal of hard to trouble shoot something approaching that online