Tensioner noise - bad AC compressor?

I have a 2001 Chevy Suburban 2500 with 91,000 on it. When running the AC and accelerating, the tensioner pulley jumps around making an awful slapping noise against the belt. This particular tensioner is soley dedicated to the AC compressor. Took it to my mechanic and he put a new tensioner on with a new belt - problem is - noise still there, but now it's even worse than before. He told me that I need to have the entire AC compressor kit replaced since it wasn't obviously the tensioner. At a price of around $900.00 to fix this annoyance, I told him that I'd wait. I have had no problems whatsoever with the AC - it blasts nice and cold and I don't have any performance issues when it's on - just that blasted noise. It does go away after I'm up to about 40 MPH, but anything under that and it's inviting odd looks from people in the Wal-Mart parking lot. . Is my mechanic correct or was I about to get hosed? Is it possible he replaced the old tensioner with a faulty one or is it possible that the compressor can cause this problem when exhibiting no other signs of problems? Any ideas would be helpful.