I have a 96 xr600r which had a strange rattle as i just accelerate of idle in first gear(much like tappets but after setting time after time I sure they are spot on)and also becomes apparent when riding at a constant speed(neither accelerating or decelerating).I suspected piston slap or the connecting rod.After stripping down the motor and checking both I found the rod end to be within tolerance and the piston just out of tolerance.So a new piston pin and rings where fitted and still the noise persists.One idea that has come to me late at night is that it maybe exhaust noise.Anyone who has experience with the xr600r may be able to help with what is normal engine noise and what is not.:think