I have a '90 f150 with a 300 6cylinder engine. When I start off in first gear there is a whistle sound and it continues thru 2nd and 3rd gear. The sound disappears when maintaining speed in 4th and 5th gear. It does not make a sound when idling. Even in neutral when I accelerate there is no whistle sound. Put the back wheels up on jacks and ran through the gears with and without the brakes applied and there was no whistle. My question is where is the whistle coming from? Where should I be checking? Checked as many hoses I could but the sound only happens when the truck is moving.