GMC Envoy air vent squeel

I have a 2002 Envoy and when I turned the air conditioning on recently, for the first time since I've owned the vehicle, I hear a very high pitched squeel. It is obviously caused from air passing over something and making a whistle sound. My question is, has anyone had this problem and how was it solved? Does anyone have specific instructions as to where in the air vent system this squeel is comonly caused? It can be heard only when the AC is on and at all fan speeds...just a little quieter at the lower fan speeds.