My 4.2 YJ whistles a bit when I'm at part throttle, I'd say about 50% into the gas pedal. The funny thing is, if I am 0-40% or 60-100% into the gas pedal it is gone, so I'm thinking it is probably something with the carb? I have a weber 32/36 installed. I would assume that a typical carb air leak would whistle all the time, not just at part throttle, but I'm no expert so I can't be sure.
Another possibility might be the breather hose on the back of the valve cover that is connected to the air box? Maybe the fitting has just the right diameter to whistle or something? Anybody know what a whistle at part throttle might be caused by?
Also, it's not terribly loud or anything, but it's enough to drive me crazy because it whistles at the throttle position that I'm usually at!