The experts on Angie's List will need more information from you before we can try to give you any specific advice. Is the smell coming from your register vents, cold air return, furnace, heat pump, air conditioning unit???
When was the last time you had anything done to the unit? Does anyone else smell the aroma? What is the make and model of your unit?
This problem may need to be diagnosed in person by a certified tech. You can find a bunch of highly rated companies on Angie's List.