The nut is loosening because the bearings behind the yoke are deteriorating. There are two pinion bearings, one right behind the gear teeth, and the other just inboard of the seal that's torn up. It's time to take it all apart and at least replace the bearings. It's also possible there might be damage to the gears if the noise has been going on for some time.
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
Love your enemies and drive 'em nuts!