Noising from the 6th gear only is almost certainly a worn pair of gears. Suprising that it is 6th gear, you would expect 2nd or 3rd. Just wear ear defenders and ride normally, when it goes "clonk" and locks up the rear wheel you will be able to identify the fault all the more easily. Has the noise got a vibration? Has it become worse during the past few weeks? Splitting the crankcase should reveal ground metal surfaces or worn gears.

have you solved the mystery noise yet?
sounds to me like a chipped gear.
only the outer metal is hardened, and they are soft on the inside, so when they go, they disintergrate quite rapidly. I hate to be a prophet of doom, but check your enigne oil for metalic particles.