I concure with Goerge. On my R65 we replaced the coupling as the 'cup' part had some wear on the 'driven' face. It cured the 'clunk' on take off.
The bike has to be high mileage for this to be an issue. Mine suffered from it from when I first had it at about 95,000 miles.
This bike is now back in my care, but its only done a few hendred miles since I last rode it. Possible 120,000 miles
Cheers
Steve