I have a 1997 4.6l with 130K. I had to replace my alternator (130amp) and while I was at I replaced my drive belt too. After, I noticed a "whine" I can't really pinpoint. Im not sure if its from a problem alternator or belt.
Also now instead of idleing (in gear) about 600 RPM, she dips down to about 200 RPM causing truck to stutter and lights to dim almost to the point of shutting off. This happens at deceleration also. I believe I bought quality parts............?

 

Remove belt and spin alternator maybe a bad bearing should spin very freely... check all other pulleys for free play as well.

 

I replaced the alternator ........again and that seemed to fix it. Thanks