I drive a 1996 F150. 164k auto. 4x4. 5.8L 351w.

This has been a problem for a couple years but it has been so intermittent I never saw it as a problem. I would really like to get it fixed though if I can figure what it is. It is too intermittent to show a mechanic.

I'm pretty sure it has happened at all speeds, but is absolutely most prominent in highway driving.

What it does is whistles/screeches, sometimes faintly and briefly, sometimes deafeningly loud and high pitched

City driving it only does it briefly before a shift, then goes away, sometimes just a chirp around 2500rpms. Pretty much only when I am accelerating.

On the highway though, is where it scares me. It gets SO loud that I cannot drown it out if I tried (and I have tried). It is much louder on the highway and often constant. (RPM related??)

The sound comes from my front end / engine compartment, but I have not been able to reproduce it by revving the engine in park and I have never heard it at an idle.

It shifts smoothly.

My hope is that its just a tensioner pulley ? What else could it be? HELP!