That is the fuel vapors causing his issue. My 04 does it, too. Actually, my Ninja 250 also makes the noise that the OP is talking about once in a while. If I raise the RPM's a bit, it will go away, as well as being related to the clutch. I think it could be a leak between the cylinder head and the carb, but I haven't noticed it the past week, so I'm not worrying much about it.