I am not sure what this is so I am on here. I have a 2000 F 150 5.4l ext cab 4x4 with 78,000 niles. The sound starts at around 1700 rpms through 2100 rpms. That is the only time I can hear it. It is a high pitch whistle sound comming from inside the dash, or so it seems. I tried to listen for it under the hood put couldn't hear it as well as I did inside the car. The noise is present all the time, in drive or park, stop or going. I have not replaced the belt on this car because it looks fine with no cracks. Any suggestions from you guys would be great.

Thanks

 

I have the same truck as you and when new mine did the exact same thing. The dealer did something around the air intake snorkle where it fits on the throttle body. I think there was a tech ser. bullentin on this.

 

thanks that narrows it down for me.

 

Yep, check the air intake tube where it attaches to the intake manifold. Mine had slipped off a bit and was leaking some air. Same symptoms as yours. Put it back on and tightened it and it fixed the problem.