alternator question

[harris145]

i bought a powermaster one wire 100 amp ford alternator from summit when i put in and started it and gave it some gas i heared a high piched whistle sound comming from the alternator so i thought it was the bearings so i returned it and got a new alternator same kind and amps and put it in and there is the sound again so my question is is the noise supposed to be there im talking it is loud enough than i can hear it pretty loud when im driving down the road. its also a new belt and i tried tighting and losing the belt. i did start it without the belt on and it stoped thats how i know its comming from the alternator any help on this would be great thanks in advance