I am having this really annoying noise coming from what seems to be the passenger side defrost vents on the top of the dash near the windshield. The best way I can describe the noise is as " screeching" sound. There does not seem to be any method to the madness. As long as I am driving, the noise will start and stop, then start and stop. It seems to last for anywhere from 5-25 seconds and will stop for the same period of time before starting again. The truck is a 2004 Silverado 1500 LS with dual climate control. It started after I had to carpool with someone to a meeting, and left my truck in a parking lot backing under the Florida sun from around 8:00 a.m to around 4:00 p.m. This is the first time EVER i have heard any noise from the A/C I have done a little research and there seems to be some decent information on bad Blend door, Vent door, and Recirculation door actuators. My confusion is that the noise coming from my truck is nothing like the "Clicking" sound described by most others. Also, as I stated before, the sound doesn't seem to be triggered by any certain position of temperature, vents( direct, floor, defrost), or driver or passenger selection. I put both sides individually on heat and it began to blow heat as it should. Does my truck have all three ( blend door, vent door, recirculation door) actuators? Any other thoughts on what the problem might be? Please help!!!