A whistle is about the best I can do to describe the sound. It's actually more like the kind of sound you would make if you were whistling by sucking in rather than blowing out. That kinda makes me think its the airbox, but I'm not sure. I never heard it before yesterday, or at least didn't notice it. It happens when riding or when the bike is in neutral. I usually here it the most rite after pulling in the clutch or letting off the gas. Tell me this is normal and I have nothing to worry about.