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Well known issue on K bikes.... the starter brush set to replace and a full cleaning.
I had the same issue on a K1

Well, i have searched with a screwdriver where this noise could come from. It looks to be louder on the gearbox case than on the engine case.
It is neat that the noise disapears when the clutch is disengaged ( decoupled ).
As it is not an engine noise, I hardly understand how a synchro could fix it.

hi men,
K engine lover ( i have a K1 too), and pretty happy with my 60000km k12RS, but a knocking noise appeared recently at idle when the engine becomes hot, disapearing when clutch is dis-engaged or when increasing RPM.
the bike runs well, without any abnormal engine noise when cruising.

Any clue ...