My truck (a 2000 GMC Sierra SLT Extended Cab 3-door, 4x4 with the 5.3l engine) makes a weird noise. It started happening tonight after leaving my mechanic after having my winter tires put on. Nothing was touched under the hood and other then jacking up the four corners and swapping tires nothing else was touched. I am hoping to find out what it may be before tomorrow evening when he is planning on dropping by my place.

The sound is a quick air burst sound. Almost comparable to the sound one would hear from the air blast when disconnecting an air powered impact gun from a pressurized air hose. It happens when the engine is running at 2000-3000 rpm (that's where I tested it) but not at idle which is indicated at around 600 rpm. Here's what Is stumping me, it only seems to happen when the interior ventilation controls are set to the defog setting and the interior fan is at least on setting 1. Changing the fans speed doesn't seem to make a difference. I have only tested with AC off so I think AC can be ruled out.

I am searching google but so far haven't had any luck and thought I might give this all knowing board a shot.

Any ideas?

Rene