Your ac compressor is ruined and needs to be replaced. You can only check the compressor by removing it and inspecting the in and out ports. My 02 Burb had the compressor go on it. Covered by warranty, though they tried charging me for putting the belt on! You need new Compressor, Tensioner, belt, and new R134a. If you do the compressor and tensioner your self, take it to a shop to get it vacuumed, pressure tested, and then refilled with R134A.

 

He is right, the ac compressor is damaged inside causing the belt and pulley to vibrate. It is frustrating because the ac still works, but the new one puts out much more cold. The compressor has a series of reed valves and one or more of those broke off, causing the imbalance.