I have the same problem with my gas pack central unit, propane. I bought the unit new 3 yrs. ago. My old, original central unit was also propane. No problem. Same propane tank and supplier. The second year of using my current unit, we started noticing that everything was dusty. Looked like sheet rock dust. I've always kept my return air filters changed and used good filters. Last year I had to dust all the walls and doors. I've had my central unit checked by 2 different HVAC men and found no problem. I've stopped using the central heat and use my fireplace. Today, the white residue is above the firebox on the wood mantel. With my central unit, I've never had a problem with the air conditioner part. I need some advice.