I had a new furnace and AC installed by a good installer. It is great other than the fact when running AC their is some noise that is caused by the velocity of the air that sounds like a higher pitched sort of howl. When it runs on heat it is quieter than the old unit (blower is apparently runnning slower. What could be done to lower the noise? Someone suggested using one of those higher density filters (3M,etc) when running on AC.