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2001 Jeep Mystery Engine High pitched squeal - HELP

This is a noise mystery for a 2001 Jeep Cherokee Classic. My mechanics are stumped. Here it goes. Generally in cooler weather but not always, a very loud squeal/whistle develops after 3-6 miles of highway speeds (55-70) only when the accelerator is not pressed. As soon as the accelerator is pressed, sound disappears. Let of the gas and it’s back. No belts are loose or worn, the vacuum system seems secure, no bearings in things like the fan, AC, alternator, etc are worn. The sounds appears to come from the back of the engine compartment and after idling the car for several minutes, the sounds stops on its own and will not come back until driven as listed above. Stopping, turning off the car and restarting stops the noise until driven as above. Help.

I had a 2002 Jeep Cherokee with something very similar to this. Do you have a serpentine belt on a 4.0 engine? Mine turned out to be a weak belt tensioner. You will have to replace the belt as well because all that squealing is probably the serpentine belt getting a lot of wear.

Yes, but they checked that out. The belt is not worn and everything seems tight and in good order. The noise really comes from behind the engine toward the firewall and not the front.