tranny whine

My dad has a 2000 SLS and he says he noticed a whine comming from the tranny. The whine doesn't change with engine rpm's, but does with speed. Tranny temp doesn't matter. While moving if you put the car in neutral and rev it the whine doesn't change. Anyone have any ideas? Car still shifts fine, just an annoying sound that we would rather get checked before something happens. My dad says the best description for the sound would be like snow tires or mud tires on a highway.