I found problem. It all started with a rear puncture. I drove bike slowly with flat rear for one mile to get repaired 1st mistake. This caused the drive shaft to break inside the diff, i then replaced diff and shaft and thought all was well. Then 6 months later i got whining noise on deceleration, this was caused by the nut coming loose that holds the yoke to gearbox shaft. The front of the rear driveshaft fits into the yoke and gives you drive. Remove the near side lower engine cover, you will then see the fuel pump, this is mounted on inner plastic cover that hides you veiw of the drive yoke but with a handlamp you can see through gap in cover to see the nut holding the yoke on. Yes it is repairable but lengthy job. If threads left on gearbox , clean or chanfer the remaining threads and use a thinner nut to resecure the yoke to the gearbox. My next repair is permanent and prob not advised but it worked forme. When i resecred the nut and yoke i welded the nut onto the threads. Bike fine now.