I have a nosie coming from the serpentine belt and pulley drive system. Not sure if it is just a belt noise or the bearing as well. If I spray liquid lube into the alternator area the noise subsides, but comes back the next time I drive. I recently replaced the idler pulley which seemed to result in a slight improvement
The noise you are hearing is coming from one of the bearings of the tensioner pulley, water pump, alternator, A/C pulley and possibly even the power steering pump if the noise is a bearing noise. Sometimes just a belt noise can be due to the tensioner is weak or the belt is glazed. You say it improved by replacing the idler pulley and the issue could be one or more listed part due to all have the same mileage on them. My recommendation is to have a mechanic, such as one from YourMechanic, diagnose by hearing the noise and using a listening device to narrow the noise to one component and taking off the belt and feeling each pulley to help narrow the failure to one of the pulleys.
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