Originally Posted by Jerz616
I dont think its out of balance. Its the magnetic pull of the magnet from the magnet near the sensors in the endbell. If you pull apart another motor you will probably see the same thing.
Rotors that have some time on them can get out of balance with the magnetic field, where the north or south pole is stronger than the other causing this problem. This is often caused by heat over a long period of time such as in the case of a used motor even if temps have been kept in check.
If the motor runs good than it is not a huge concern, but your options on fixing are either new rotor or trinity offers a rotor zapping service now that re-saturates the magnetic field of the rotor back to like new condition.
Hope this helps.