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.

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