ok no brushes then...
you'll probably want to have motor checked for a bad bearing or possibly something coming apart inside it, or it could be as simple as some trash got inside it maybe? Make sure all the cables are securely connected and corrosion free.
i can think of maybe one other cause but that would affect the motor performance and would require checking the motor circuits and how they draw current on each leg of the power circuit, a diagnostic tool would be required for that to monitor the electrical operation between the controller and the motor.