I live in Northern Arizona and recently began using my furnace as the temperature drops. I notice that while it is running, I hear a high pitched whistle in the rooms near or below the unit (it is located in the attic). Forced air heat, 3 year old home. This was also a problem when the unit was first used and it took a technician all of 5 seconds to fix the problem. Is there some adjustment I can do myself? Thanks.