Never actually "heard" an alternator make any noise during operation. Make sure it is the alternator doing so and not some other component...although bearings mounted improperly may contribute to the noise.
Also make sure it is installed correctly and aligned properly with the serpentine belt...any belt residue (dust particles) showing up on the alternator housing?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car