I guess I wouldn't get too worked up over it if no noise was present with the old belt. They'll sometimes sound wierd when unloaded. If you're really concerned, you could remove it and try getting some oil to go into the sealed bearing, or just replace it, it's available seperately. Usually the tightener will junk out before the pulley itself will, and the new tightener comes with the pulley. You may want to check the fan hub bearing, and also the idler pulley that's on the fan hub bracket,( the ribbed pulley just towards the passenger side of the car from the fan hub). Of all the pulleys on the car, I'd say the fan hub is the most prone to wearing out, probably a toss up between the fan hub bearing and alternator bearing noises.
ps It may be more understandable to refer to the belt as the serpentine belt, I know MB calls it the poly V belt. Serpentine belt makes it alot clearer what type of belt you are dealing with.