2002 Chevy Tahoe Squeaky Belt?

Order of changing parts. I bought the Tahoe in Sep '08. Person had bought it brand new and frequently changed the oil and did proper maintenance on it. They took it to a stop and go place to change oil frequently. When I bought it, it had the squeaking noise. They took it to back to place where they change the oil and they said the belt was new and that is why it was squeaking. I took it to my mechanic after purchasing it and he said I had a bad water pump and a old belt on the AC. He changed both. I still had the squeak. I then changed the tension pulley and it continued to squeak, so I put the old one back on and took the alternator off. It had a squeak to it so I took it to AutoZone and they said it was bad, was not producing a charge. They sold me a new one. Put that on and it continued to squeak. I then changed the Idler Pulley and it continued to squeak. I then put a new belt on and it stopped squeaking. I was happy. The next day I went to the store and came home nothing. My wife left to get a coke and on her way back it started to squeak. It was only squeaking when you pressed on the accelarator. Now it is a constant squeak. Any help would be appreciated. Thanks