If your bearings are being loud. First try and use some form of spray lube to silence the bearings.

Spraying oil into the bearings while the tires spins is the best method. Make sure if any oil gets on rotor to clean off. 

Hearing a "knocking" noise is never good. If this happens you might have a failed rear bearing.

This is rare but anything is possible so never say never.

  1. To check if your bearing is lose and causing a knocking noise you need to first look at the axle that does not have brake or rotor.
  2. Once looking at the axle and bearing you will see black retainer clip.
  3. Grab the axle and see if any play is in the bearing by moving back in forth.
  4. The black retainer clip is a good indication to watch for movement. 
  5. You will see the entire bearing outside move within the rear wheel.
  6. If this happens your bearing has failed and will need to be replaced.