Um, just a bit of warning here ....
A couple of years ago, I had the same noise coming from the alternator on my '91 Jimmy. Now, it had been doing it for a few months, and I just hadn't gotten around to getting the bearing replaced (I was assuming that it was the rear bearing). Well, I guess you could say I waited too long to do it, because one night on my way back from school (on the interstate), the front bearing seized and spun the outer bearing race in the front housing of the alternator. The entire rotor assembly ended up walking out of the front of the alternator, which instantly killed the truck, shorted out some of the wiring harness, and started a fire under the hood (the wiring insulation was burning). Thank goodness I ALWAYS have a bottle of Diet Coke with me, otherwise it might still be burning by the side of the road. Excitement like that I can do without....
So, I guess the warning is, if it'* been doing it for awhile, you might want to get it checked out before you take it on any long trips or anything.