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.

this is the newer NDR035 D831N06177T ac motor

winding noise eh?
does it sound like clicking, speeds up when you accelerate?
is this an AC motor or DC?
if a DC then check the armature for unusual patterns on the commutator where the brushes ride.
if the motor has been stalled any the com bars will raise up and cause this.
the only remedy if this is the case is to replace the motor.