XJ700 running engine squeals like starter motor is engaging

Hi, I'm new to the forum, forgive me if this issue has been mentioned before otherwise please help!
My '85 bike had been stored and shows low mileage of 18k miles. When attempting to start if it doesn't fire up quickly I get the rattly bucket of bolts noise like the starter clutch exploding. Following full service it now starts quickly enough to not reach the rattly noise stage. Great I thought and started to ride it. Recently and getting more regular is a loud squealing noise coming from the starter area that varies with revs. I've checked the alternator stator and the brushes seem quite new. I've turned the motor whilst holding the starter idle gear via the rotor bolt - one direction turns the motor over as expected and the idler turns, whereas the other direction the idler slips expected, so low speed mechanical turning seems ok without any weird noises or jamming. I've eliminated the starter switch and solenoid by starting the engine and then detaching the supply cable to the motor before riding off - still happened. I think next check may be to remove the starter motor, plug the mounting hole, bump start it and ride off...
Is it possible for a slippy starter clutch to lock whilst the engine is running and then turn the starter motor even when electrically disconnected? If so how, is it falling apart or the roller springs broken up?
Any pointers or past experience really appreciated as I'd rather not strip the engine!!
Cheers
Dave

 

Update
I stripped the starter motor and found that the one brush that bolts to the outer casing had a crumbled plastic insulator block. Chunks of the insulator had moved along inside the motor and got caught up between the armature and field magnets. The commutator end of the armature was badly dirtied with brush dust, resulting in shorting. I found a new brush set with holder on Ebay and fitted it following a thorough clean-up of the armature. Bingo, it fired well without any weird noises and didn't try to start turning during a hard test ride. Seems the starter motor was shorting out inside and trying to turn whilst the engine was running somehow! never came across that before but hey, we learn new things all the time with classic bikes

 

This doesn't quite make sense does it? Starter motor turns when the solenoid pulls in, or if the engine turns backwards. Whether a totally failed starter clutch could cause it is a question I can't answer, but essentially, no power to starter, no spinny