My theory is the alternator's monkey nuts bouncing back and forth on the drive dog. Before the engine gets past ~2500rpm there isn't enough speed/momentum to hold the nuts against one side of the dog so they're able to bounce back and forth in the cup. You get the same noise characteristics from the alternator's fan if the shaft nut is loose, which is what got me to thinking all this...
I would like to think if you made the alternator to drive dog connection tighter then the noise might go away, but by the drive system's design it would be difficult and probably not worth the hassle (you'll know what a pain it is if you've ever tried to r&r the alternator with the transmission still in place).