Noise from alternator

2007 Grand Caravan 3.8L 100K mi.
New noise chirp-chirp-chirp Sometimes an intermittent rumble at idle seems to be in synch with the serpentine belt especially when the A/C is engaged. I changed the serpentine belt which did not look bad and did not help the noise. Noise seems strongest at the alternator when I use a stethescope. I noticed the pulley seems a little loose with the belt removed, but the bolt is tight. This alternator has an overrunning alternator decoupler (OAD) pulley so maybe the loosness is normal. The bearings seem smooth when I spin it with the belt off, and it charges fine. Just the noise is bothersome. Has anyone seen this? Do I need to change out the alternator ($300) or just the OAD pulley ($50)?

Since nobody replied, I figured it out myself. It was the alternator decoupler pulley. I changed it out and all is well. I needed a special tool kit and was able to change it out without removing the alternator from the vehicle which would have been a big job.

Many chirp noises from belts can be solved by "rotating" your belt-- take it off and put it on the opposite direction- wear patterns develop and this changes those- you did the same thing, but by changing the wear pattern with the pulley- or maybe you did reverse the belt direction and did not realize it- anyway, I prefer my way because it is cheaper - as in $0 dollars...

Did you try that first by chance?