Rear hub noise?

I have a little 36v Voy that going good but its starting to make a noise from the rear hub. It can best be described as the sound that a crystal glass makes when running your a wet finger around the edge, not as high pitched as brake squeel and stops when you apply the rear brake. I havent had this moped too long and havent taken the rear end apart yet. I tryed adjusting the brake but even adjusted so far where it doesnt engage fully it still makes the noise sometimes. It is louder with a heavier rider on it. Any thoughts? Thanx