It is pretty hard to diagnose a noise over the internet that at least 2 mechanics can’t pinpoint with a stethescope but if it was my car, I would try this: Remove the serpentine belt. start and run the car briefly. See if the noise is there with the belt off, If the noise is there, then it is caused by nothing driven by the belt. Look elswere. If the noise stops with the belt off, turn every pulley by habd looking for any roughness or play.
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