Kinda pointless now. The sound suddenly went PFFfffffffttttt.... last Wednesday and hasn't returned since. Really makes troubleshooting difficult.

The truck is behaving normally though. We had to go on a 3.5 hour + round trip last Thursday and we were towing our 23 foot travel trailer for the return portion. The truck behaved as it normally would and there hasn't been the slightest indication of any noise since.

I'm wondering if since the truck was sitting for a few weeks prior to this noise maybe a valve got stuck and finally freed up after being used.

The A/C definately appears to be off when in defrost mode (needed this mode this morning btw, -7.5 C) as the light toggles on and off and there seems to be the sound of A/C engaging. With my CR-V when you switch to defrost the light comes on and you have to manually turn it off.

Thanks for all the input though. At least now I know where to return to get some good resources on the subject!

Rene

P.S. You guys were much more helpful the the GMC Sierra Truck Club on Yahoo!