This may be a completely noob question; however, with these warm days we have been experiencing I have noticed a very high pitched whining noise coming from my fuel tank at very random times throughout The ride. I know the sound is from pressure inside the tank because as soon as I open it the noise stops completely. It generally last 15-20 while starting her up, then stops for a couple seconds then another ten seconds and does it for nearly a minute after I turn it off. Once I thought I heard it while riding; but, my exhaust is so loud I couldn't be sure that's what I was hearing. Is this a normal ninja thing; or, should I be concerned?