If you have a centre stand on the bike, put the bike on it and carefully run it in gear, say 1st, 2nd and 3rd and you might be able to hear where the noise is coming from I.e. chain/sprockets, rear wheel or engine. You can also check that the chain isn't catching anything. There are a number of other things you could have a look at. Check for excessive forward and backward movement of the rear sprocket that could indicate excessive wear of the cush drive rubbers. Check for side to side and verical movement in the sprocket carrier itself that would indicate wear of the carrier bearing. If these are all OK, then remove the gearbox sprocket cover and check that the sprocket nut is tight as already suggested. Also check to see if there is any obvious side movement in the final drive shaft itself (bearing failure). Finally with the bike on the centre stand (if you have one) or with the back wheel off the ground, turn the wheel by hand and see if yoy can hear any unusual noises coming from the engine that could indicate problems with the gearbox. Hopefully you should be able to find by elimination where the noise is coming from and get it fixed. If the noise appears to be coming from the engine/gearbox internals I would suggest not riding the bike as further (and more expensive and potentially dangerous) damage could occur. Hope this helps.
