There is a one-way clutch in the starter drive gear. It sounds like yours is getting ready to fail completely. It's not terribly hard to swap the drive assembly out, but this might be a good time to get a rebuilder to freshen up the old starter. Be sure you get the correct one for the flywheel gear tooth count, although I don't think the 6 and 12 volt starter drives will interchange due to a shaft diameter difference.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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