I have a new 09 Silverado. Running the AC on high when I first get in the truck it sounds normal. After about 30 minutes of driving with the ac if I turn the blower back on high it has a very noticeable buffeting noise. I've checked the evaporator drain and it's clear, thought it could have been a build up of water. This happens with recirculate or outside air. Let it sit awhile and it's back to normal fan noise. Ideas? Thanks.