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Remove belt and spin accessories by hand to feel for any roughness or noise (water pump/alternator/power steering pump/a/c). Check the belt and or tensioner . Belt is the biggest suspect and if the original probably is the cause and a cheap and easy fix.
I have a 99 4.0, I've spent weeks tracking down the noise. I changed water pump, idler pulley, tensioner, belt, and finally tracked it down to my power steering. I spun everything without the belt, and even used an small electric hobby starter to turn at high speed ....still nothing, thought i heard noise from water pump that's why i changed it out. I finally got fed up one morning after getting off work, I place a small broom handle carefully to the back of the power steering pulley while running....I know its not safe, nudged it forward, the high pitched chirp disappeared, after letting go, it was back. I replaced pump and all the chirping is gone. I tried everything to locate the noise, but could not isolate it.
Rezvani's Latest Post-Apocalytic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.