I acquired this clutched, hepa model which had a severe screeching noise from down below. I stripped it down and found the brush bar was seized. I stripped and checked the clutch which seemed ok. I fitted a new brush bar and it worked ok for a few months until the brushes burnt out. I replaced the motor and now the screeching noise reoccurs when the clutch is engaged. It may go ok for a few moments ok, but then suddenly starts screeching. What sort of faults can a clutch develop?