That is probably the overrun 'clutch' in the rear axle. It's purpose is to ratchet so the inertia of a heavy, spinning pto implement can't push the tractor after you put in the clutch. (US Yanmars have this feature, not the grays).
It's not really a friction clutch, it's like a one-way ratchet. The sound is identical to the clicking of a bicycle when you quit pedaling - except tractor sized.
Putting in the clutch and coasting backward in a forward gear (or forward while in reverse) will cause this ratcheting sound. Like when you push on a stump then roll back a couple of inches.
Relax, it's harmless. Same as coasting downhill on a bicycle, click click click.
It was the primary tractor for all of the 'tractor' work on the ornamental tree farm when established 10 years ago. That big clicking sound you are hearing is normal. Mine was bought well used and was developing more problems each year I used it. About seven years into using it I was in a position to "up grade" and was researching compact tractor brands online, I found tractorbynet, hoye tractor and posts from california on the 240 yanmar. I was gunna sell the poor thing off but with the knowledge found on TBN that changed.... Then shortly after that while looking at tractors and for the price of "If I load it, would you haul it off" I scored on a parts 240, engine dead, but with many other donor parts plus a loader to make my 240D come out of retirement.... Only thing is I'm gunna have to retire to have the time to do all that
