Whistling/rubbing sound
So every time the rear wheel makes a rotation, a sound that strongly resembles brake rub emanates from what I think is the hub. I've triple checked that the brakes weren't rubbing, and removed the fender to rule out the possibility of fender rub on the tire.
Also, a strange knocking sound can be heard when the rear wheel is spun slowly, as if there were rocks tumbling around inside the hub.
Are these sounds related, and what can I do to eliminate them?