If it's a "chirping" noise, then you don't have the belt tight enough.
Spray hairspray on the belt if it is making the noise or replace the belt or the pulley if its making the noise.
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The belt is slipping.
Nine times out of ten it is a worn serpentine belt. Tensioners are also know to make a rattling noise congruently. Changing your belt and/or tensioner should solve your problem.
Assuming you have already checked the obvious, the serpentine belt tensioner and belt. A common cause of the squeal in 4.0 Ford engines is the camshaft sychronizer (takes the place of the distributor on DIS, distributorless ignition systems). It sounds very similar to a belt squeal and is often mis-diagnosed as a belt being loose or worn. The noise can be intermittant, but if the noise is there when you have the hood up use a simple mechanic's stethoscope to help pinpoint the source of the noise. == == The most likely culprit is a serpentine belt or the belt tensioner. Try replacing the serpentine belt. If the noise persists, replace the belt tensioner. See the related article "Serpentine belt and Tensioner FAQ for the Ford Explorer all models"
It may be the serpentine belt. Sometimes when you purchase an "on the cheap side" serpentine belt, it has a tendency of making noise even when the tension pulley has been change. Try purchasing a more expensive belt. The diamond back serpentine belt may be the best solution for this problem.
Check your Serpentine Belt and pulleys. The Serpentine Belt maybe in need of replacement.
the belt is ether too old, or a faulty component that is being driven by the belt.
Remove the serpentine belt and check the belt and pulleys for damage. While the belt is off, turn the alternator by hand listening for the noise.
Check the allignment of all the pully's.
If you find cracks in the ribs and a certain audible noise from the AC