I've fixed many the motor by getting the gunk out. Sometimes all it takes is a piece of sand to get in there and make a horrendous noise and slow things down.  

Try blowing or sucking (with a vacuum) the area where spindle sits in the bearing.  

This is sometimes the case with switches, as well.  You need to operate the switch while blowing or sucking the air through. 

For those bearings at the ends of a brush roll : I've seen where so much hair wrapped around them that the hair got into the bearing.  Get a tweazer and pull that out.  Again use compressed air or a strong vacuum to move air through while operating it.  Then lubricate it generously with a quality oil that will act to "wash" out the remaining particles.  

For power nozzle motors or any appliance motor; I've seen the motor housing get filled with lint or hair.  Get a tweezers or needle nose and pull all that out while holding a vacuum hose to the area.