Noises problem
2001 Ford Mustang V8 Two Wheel Drive Automatic 107500 miles
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Engine accessory belt squeals upon starting, have replaced idler pulley. It seems noise was coming from there. by holding a screwdiver to to bolt the noise was originating there. Idler pulley replacement did nothing to quiet the squeal. belt looks good. Belt dressing spray does not help.
What is the next step reccommended to solve this problem?

Replace the tensioner and the belt, save yourself the headaches! Also, with belt off, check the A/C pulley, make sure it spins freely, no binding or grinding!, can be the pulley bearing inside...