I have that issue as well, the gasket on my gas cap is crusty & cracked. If it makes that noise, push the gas cap down and it it quiets, then you'll know that's the culprit. I could not find place to buy a replacement gasket, so I ordered a lightweight gas cap from slingshot racing. I looked at kawasaki parts house.however, I couldn't find a part # for the gasket.Hope that helps.
1. SLK 230K AMG SPORT
2. VW RABBIT 2.5
3. 08 Ninja 250R Track Prepared