A pinion nut that's loose is an indication of bearing wear. Tightening the nut is only a temporary fix- - - -it will get loose again in a very short time. As the pinion bearings continue to wear, they will get noisy and eventually fail altogether. Be on the lookout for a hum or whine that changes pitch as you go from pull to float to coast. If you haven't run for very long with loose bearings, it might be possible to swap them out before you get major damage to the gears. It's sort of a crapshoot, though!
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!