Bicycle Mechanics - 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?

The whistling sound might a rubber seal over the end of the bearing that rubs against the outside of the hub shell. If your hub has such seals, fold them back and insert a drop of light oil, such as Tri-flow or Boeshield T-9.

The knocking sounds like it might be a bad bearing, either in the main hub or the freewheel section. Does the wheel wobble sideways when you put pressure on a side of the rim? Does the knock appear only when coasting or under power? The sounds have different causes, but they both may warrant replacement of the hubs or wheels.

The knocking sound probably is bearings - does not necessarily indicate damage but most often indicates too little grease, or grease that has broken down. Overhaul the hub, lubricate the seals and then check for any remaining noises.

Another possible cause of the rear wheel's noise could be a loose bit of metal from the rim manufacturing process, although that'd probably be more of a tick tick kind of sound. If in the hub I'd suspect the freehub if the hub bearings are already properly adjusted.