I own a Maxxum now, that is only a year old, but I had a much older Kota that had build up from the carbon brushes between the plates of the commutator. This was after years of use though. Cleaning this out with an ice pick rejuvenated the motor and stopped the vibration until my Maximizer died.
If there is a lot of in and out play on the shaft, the brushes could be hitting the edges of the commutator. I am convinced that I aged my older motor by not picking it up every time I motored slowly to a new spot thus pulling back the prop un-naturally and causing wear where there was not bushings. If you slowly spin the prop by hand and look at the groove between the prop and the housing, you should be able to see if it is out of line. You should also be able to hear if something inside is catching.