I have a 2006 Monte Carlo with the 3.5 engine---its making a squealing noise--it has had a new water pump, new belt tensioner assembly, and new grooved pulley on the front/top , and still makes the noise. I installed my old belt and it quit for a few hours and then started again, its very loud, you can hear it going down the road at 60 mph, there is two other pulleys with bearings in them, that is my next step, one is by the alt. and the other is under that pulley. Could it be something else?

 

It could be any of those idler pulleys, belt tensioner, water pump, alternator, or even the belt itself. Take the belt off and spin all the pulleys, alt, etc by hand and see if any feel funky or have slop.