Do you have a cogged belt or a regular flat belt? Cogged belts sometimes creates bad harmonics. Also, the belt should set down in the pulley, not up flat or "above" the pulley. Is the pulley straight or does it have any wobbles or is it bent in any way. If the tension is too high it could "pull" the alt and create a squeak. Is the pulley inline with the main pulleys? My alt is on the pass side and I have never had any squeaking even if I hammered on the loud pedal.
An old "remedy" is to use a bar of soap and apply to contact surface. You can do this with the car running, just be careful. It isn't a total fix, more like a band-aid, it will probably only work for a short time.
Let us know how you fixed it. Others may have similar problem.
Good luck and have fun.
Cheers, Jer