Pull the motor apart as suggested above and check the bearing/bushing. I've had a couple that got noisy and all they needed was the bearing and bushing oiled. Be sure to get a seal kit for the TM before taking it apart as you want to use new seals when reassembling the TM. Also, be sure to mark the magnet housing (white pen/paint) so it goes back right where it was removed, if it gets flipped the motor will run backwards.
Not sure how your MG goes together, but on the MK's I have worked on they need a cardboard tube brush retainer to hold the brushes open, so the armature can be installed. If the brushes are stuffed way down in the motor housing and impossible to reach, you probably need the retainer.
