its probably the exhasut pipe ,the internal baffles come loose when the spot welds break inside and rattle like hell,if it sounds like its inside the engine then the cam chain tensioner may be faulty get it sorted before the cam chain jumps a tooth ££££££
to check the exhaust have the bike cold and turned off and hit the side of the exhaust at the back with a rubber hammer it should replicate the noise if its the exhaust that is the problem.
adam bike doctor(of milton keynes motorcycle centre), December 2010
