The motor armature (rotating spindle) is arcing across the carbon brushes causing the carbon brushes to 'flick' in and out of their chamber. Eventually, once the carbon brushes are worn down, the motor will pack up altogether. Unless it is possible to clean the commutator (the part of the armature that the carbon brushes contact), i'm afraid this is a permanent fault that can only be cured by replacing the motor. The commutator is either rough through lack of use/storage in a damp environment or it has become eccentric through overheating of the motor (caused by blocked filter screen).
Gordon, January 2011
