Thanks blokes for sharing your experience. I may say that I am mechanically minded and have had the same thoughts. The whining sounds similar to worn diff on a car. Chain is not too tight, I can't believe that the gear box can fail after 24 000 km on the clock (but you never know!). I think I will check the rear bearing first as I noticed that the seal didn't look great when I changed Tyres 3 months ago. Anyway, the winter is knocking on a door, there will be plenty of time available to concentrate on the problem.