My 2002 HO 6 speed will make a flutter sound and vibration, like a flat right front tire, when pulling a steep hill WOT in 4th or 5th gear. I think it is the plastic air horn connecting air filter and fender air intake but not sure. Backing off the throttle slightly will stop the noise and vibration. The restrictive gauge on air filter doesn't show any restriction history. I've thought about removing the plastic air horn to see if that stops the noise. Sure would appreciate answer from anyone who have solved this problem. After 6,000 miles problem seems to be improving (less frequent).