Auto Air Conditioning & Heater Repair: AC noises, tensioner pulley, gm pickups

Question
i have an 02 silverado. the ac system has started making noise when it's turned on. I've replaced the tensioner pulley and the belt. it only seems to make the noise when the truck is under a load (ie. taking off). I've watched the system when the ac is engaged and it's making the noise and it looks like the tensioner isn't holding the belt tight but I've tried three different tensioners from different parts stores all with the same result. have any ideas whet this could be?

Answer
This is a common problem on GM pickups, your going to need a new a.c. compressor! It is dragging too low on initial engagement, thus pulling the tensioner too far. I have repaired  a number of these problems exactly as you described. Every one has been a compressor.