I've been hearing this lately when warmed up and idle...
060.flv video by iplayloudly - Photobucket
It doesn't do it when cold. You can hear compressor kick on and the noise speeds up a little, but I don't think it has anything to do with the compressor. I hear the noise if the air is on or not.
I thought it was a pulley but it sounds slower than the speed of the serpentine belt... I hope its not a timing belt pulley.
The truck seems to run fine otherwise.