What is causing belt noise in my 2003 ford taurus?

Just replaced my alternator and a few days later started to get belt noise. Replaced the belt and idler pulley since they seemed to be the top causes and the noise went away for about a day. Noise is now back and I can't figure out what is causing it. It seems the next common cause is the belt tensioner pulley but when i take the belt off the pulley spins freely without a problem. The ac pulley seems to spin freely as well. The noise gets worse as the rpms go up and I'm all out of guesses on what it could be.

Maybe it is the alternator after all you didn't get it until you changed that item.

An even more common cause is the belt, either stiff, worn or contaminated with oil. A lot of replacement belts also suck these days. The best ones I have found are the NAPA belts. They are made by Gates and are pretty good since a lot of places don't stock the better brands any more.

Troubleshoot. Remove the belt and check all the pulleys for side to side play.