I have very little air coming out of the defrost on our 05 QX56. Most of the air comes out of the front vents, even when they are turned off and the heater is set to defrost. Further, behind the clove box I get a tapping/thumping noise. I took it to a mechanic and he advised just to leave it alone...that it was a "vacume problem". Anyone have a similar problem and an in-expensive fix? Thanks