I'msorry but I can't answer your question as I have the same problem. I've tried releasing the motor from the cage to clean the bearings and check the rotor, but it is virtually impossible with home tools because it is so tightly screwed in. however, one possibility is that their could be grease deposits around the inside of the casing when you remove the motor housing.
did you manage to find a solution, as a new motor can be over a £100?
mike, August 2009