My 2001 F150 with 255xxx miles has a squeaking under the hood. It is only present while the throttle is pressed while driving. It is loud enough to be heard in the cab, but not over the radio. It changes with engine RPM. It will not squeak while in neutral at any engine RPM. As soon as you lift your foot while driving it vanishes. Turning the A/C on/off doesn't change anything. The engine is a 4.6L V8 with the 4 speed automatic transmission, all original components as far as I know. The serpentine belt is brand new. No drivability problems, no CEL, with a mode 6 scan tool I do get a code for the EGR being open too much.
The squeak is never there on a cold engine. If it is under <50 F outside it never squeaks no matter what the engine temp is. If the ambient temp is > 50 F it starts making the noise once the engine temp gauge moves to the normal range. I've looked under the hood while idling and the none of the pulleys are wobbling and the tensioner doesn't jump when you open the throttle.