[Marshall V. Daut]

The poster says the starter hums. If the Bendix drive were locked against the ring gear, you'd hear nothing. Not even a clunk. I suspect that a Bendix gear bolt has sheared off, allowing the starter shaft to spin = a humming sound. Put your palm on the side of the starter and depress the starter rod. If you hear and feel a hum, that tells you the Bendix drive is not engaging, either because of a sheared bolt or a broken spring, although the latter would also produce a clunking sound as the drive spun and the broken spring clattered. In any case, it would be best to remove the starter and examine the Bendix drive and spring. A broken Bendix bolt will be obvious.
Marshall