Just some suggestions try it and see if it works for you.
If you check some of the other Kawi forums it is common amongst the Ninjas. One thing you can do if the problem really annoys you after you have stopped riding is open the gas cap, let the pressure out, lock it back and put a cool wet towel over the tank. This will cool the surface temp on the metal some degrees, in turn cooling the fuel ( might take a couple of applications) get it into a shaded area or buy a cover that just covers the top half of the bike to provide some shade. If you have been riding hard and the engine temp is high. the radiant temp from the engine will keep the gas tank hot. Cool the engine as much as possible. This may work for some, however i found that gas has a thing of its own. Even when it is cool the vapors may still build up and you get a weak whistle.