It could well be the usual issue of a plastic tyre cracking that afflicts these motors. If I may quote B&O guru, Tim Jarman...

It is usually caused by the magnetic ring for the FG servo just above the commutator inside the motor. There is a plastic “tyre” that holds it in place in the motor and these age and then crack - the loose end gets mixed up with the brushes and make a noise. Normally just removing the offending loose piece is all you have to do, the ring seems to stay in place by itself. To get at it, remove the bottom of the motor, unsolder the PCB and then take off the brush plate. It is much easier to put it back together if you take the pulley off first, then you can insert the rotor into the brush plate before sliding the complete assembly into the stator. Be sure not to lose any of the spacers and washers on the motor shaft, their positions are important. A tiny spot of oil on the two bearings is also a good idea while it is in pieces.