Yup, know the sound your talking about, almost like a detonation when on the gas hard. What it is, is the air gap on the flywheel/stator, can't remember what the clearance was but your kawi dealer should know or be able to find out. I had the same problem on my 05, the 06 didn't do it for whatever reason but I do know that it doesn't matter what year 05, 06, or 07 they all have experienced the problem. There is a thread on thumpertalk in the Kawasaki 2 stoke section, they may have the air gap clearance on it. This isn't a new problem and you don't hear much until somebody says hey ai've got this noise, then its yeah I 've got it to. I don't think it hurts performance any it's mor annoying than anything, although, maybe it does hurt performance.
Good luck.