With intermitten grinding noises, lots of times the engaged end of flywheel teeth get worn down making the bendix have to reach farther to engage the flywheel teeth, and since the bendix is already spinning you hear this grind as it is trying to engage. It would be highly likely that a portion of the flywheel teeth are worn down.
Four cylinders are real bad on this because they really have a "favorite" place where the flywheel usually stops since there is only a few places where there is a piston coming up on compression-thus stopping the flywheel at that position when you turn the key off -so those teeth repeatedly are always seeing the engagement of the bendix, thus there is a definite worn groping of flywheel teeth-sometimes requiring new starter ring (or flopping the ring gear around providing a fresh set of tooth ends) or another flywheel. If the clutch part of the bendix unit is worn out, the starter motor will just wind and wind without throwing the gear into the flywheel no grinding noises here. I have never witnessed a wornout bendix gear-though it could happen.
