A/C Belt Tensioner Violently shakes, any ideas?
 

I have a 2003 Silverado 2500HD, 6.0 and heard a violent knocking sound when reving motor. Today the A/C belt fell off and the knocking went away. I bought a new belt, reinstalled and knocking came back. Upon investigation I could watch the tensioner rocking a little as the engine idled but when revving the engine the tensioner goes nuts and the stop on the moving part racks against the stop on the cast base. I removed it and you can see where it had been hammering for quite a while.(I just bought the truck) Suspecting bad bearings in tensioner I replaced it. Same issue with the brand new one though. If you watch the belt when idling it runs smooth and no visible vibrations, same when revving but the tensioner racks back and forth violently. Neither the A/C compressor pulley nor the crank pulley show any visible vibration or being out of round. Any ideas??

replace teh A/C compressor I would think.

Well, here is the deal with that. Ice cold air... indicating the compressor itself is doing what it is supposed to. No refrigerant leaks, etc. I looked at a parts breakdown of the compressor and the pulley(which would be what I am assuming to be out of round) is part of the A/C clutch assy. So in essence replacing it would be the way to go? Any special tools required getting the A/C clutch off? I wanna avoid replacing the whole compressor, only 80K on the truck and huge difference in price of a/c clutch and whole compressor. Thanks for the reply.