If it's coming from the cylinders then it's almost certainly piston slap. The 600 has very short piston skirts, so short I asked four seperate mechanics what the bloody noise was before letting it go and just sodding riding the thing!
On the downside it did expire into a ball of flame at brands hatch about six months later
I have a feeling that was more to do with being revved to 110% for every second of the race though!
Cheers,
Ali