my trusty chinese 1/2 hp bench grinder, bought at a pawn shop about 25 years ago, has finally packed it in. It's not jammed, so I suspect the capacitor, and I'll check it. But ... it's been running louder and louder for the past few months and today was REALLY, REALLY LOUD before it quit. Not a whine, kind of medium-pitched, but LOUD. What causes the noise? 