probably the piston inside the front cylinder that's "tilting" a bit. but these bikes can grow old with that. a way to fix it is to put 750 cc cylinders on the engine. you would have to modify the piston to prevent it from hitting the crank shaft. my M600 had the same problem. i had to adress it, because it was burning oil and eating spark plugs

@@mennokleene3321 took my monster apart, had the same sound, pistons skirt all scrached up, cyllinder scratched aswell. Going to replace with newer 600 2000-ish piston/cylinder assy.