So you're absolutely sure it's the cam chain making the noise?
I ask because I know of a different malfunction that you've just described to perfection. It occurs when the rubber bumpers that make the connection with the "dogs" on the backside of the clutch basket fall apart.
The clutch basket is connected to the engine's primary drive gear (and the accessory drive gear for oil and water pumps) by "dogs" which are short round metal pins that slot into holes in the primary/accessory drive gear assembly which are surrounded by hard rubber. For unknown reasons some bikes experience failure (disintigration) of these rubber pieces and the clutch dogs end up rattling around in the now-rubberless holes in the primary drive gear. This is especially noticable at idle and less noticable with a load on the clutch. The remains (little bits and pieces) of the rubber bumpers will be found in drained engine oil or on the engine oil screen.
Read this section and tell us what you think:
http://faq.ninja250.org/wiki/What%27..._the_clutch%3F