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

I have replaced alternator three times with 110-amp alternators, have same problem, low voltage at idle, blower fan slows down radio sometimes flickers and have had it to shut off at idle and not enough voltage. I was told when the alternator whistling it is the diodes fighting each other. I don't want to pay $350.00 to $400.00 for a Hyundai OEM alternator. Any thoughts on the problem?