Low idle charge, at times alternator at low-speed whistle high pitch sound?

I can hear the shrill squeal, from the alternator from inside the vehicle. It doesn't do this all the time, but quite a bit of the time it does. When it happens my A/C blower fan slows down and when I accelerate it speeds up. My original alt. Bearings went out, so I purchased one online from a manufacturer that only deals in alt. And starters. After I installed it I was told there were smaller pulleys at a later date to help with low idle voltage. The distributor told me when it was making a loud squealing noise it was the diodes fighting each other. They did not have another alt. In stock, so they refunded my purchase price. I then went to AutoZone and purchased a $189.00 re manufactured alt. After a couple days it started again. At idle without a load, it puts out 13. 30 volts, when I rev the engine to 2,500 RPMs it puts out 14 plus volts, at idle with A/C, light, blower fan and condenser fan kicks on. It puts out below 12 volts or a little less at idle. With engine revved up it does fine. Didn't have this problem until I replaced the original alternator. I understand that the computer controls the alt. Functions, as not to overcharge or under charge. Sometimes I crank it up, it is fine but after driving around town it starts again. I purchased a new serpentine belt, thought that might help, but the same thing happened. For some reason it at times it fails to put out at idle speed. My engine at idle is turning 550 RPMs in gear, 800 out of gear. I am very frustrated from dealing with this ongoing problem. A new OEM alternator for this vehicle is over $400.00. I would not like to purchase one and still have the same problem. Hope you can help.

Yes, the AutoZone part # is 11020 110 amp. My 2.7 v-6 engine has 1 serpentine belt with a tension pulley. Sorry I can't upload the whistle noise at this time. It should not be a bearing because 2 alternators from different manufacturers alternators did the same thing. I'm wondering if my idle speed is too low or I have the wrong belt for the smaller puller. The belt seems to be tight enough. Have you had to deal with a problem of this kind in the past? Could the temperature sensor have an effect on the computer telling the alternator when not to put out certain voltage at low idle. I have noticed on cooler days it is not as bad. Just a thought.

Thank you for you patience and time, I have already replaced the idle control valve, cleaned the throttle body and replaced the throttle position sensor. When it makes this sound and the idle voltage drops, as I accelerate and it goes through the gears is when the shrill noise is louder. I will adjust the throttle cable to idle it up a few rpms I will idle it up a bit and replace the belt to make sure I have the correct belt for the smaller alt. Pulley. If that doesn't help, guess I will have to live with it. Thank you so much for your help.