I have a 2001 Ford F-150 with the 4.6L modular V8 engine. This morning I noticed it was making a high pitched noise when accelerating hard. This is audible in the cab of the pickup truck. Upon further investigation it seems only to happen above 3800 RPM as indicated by the tachometer.
I ran the truck with the vehicle in park and was able to replicate it. It happens revving the engine in park with the A/C off. It's extremely loud if you're in the engine bay. Everything sounds completely normal till you get to high RPM, then it is almost deafeningly loud, like a siren or alarm.
I made a video. The audio quality is not excellent, but you can clearly hear the high pitched noise from the engine bay as I open the throttle of the engine.
After removing the fan belt, I discovered the idler pulley was difficult to turn, the tensioner pulley had radial play, and the waterpump had radial play. I have replaced all 3 of those with new components but there is no change in the noise.
During replacement I noticed there is a small intermittent vacuum leak. Is it possible that a vacuum leak could be creating this noise? Almost like a whistle?