Loud Squeal exists when gas pedal is NOT pressed

[RotorHead Dave]

I have a loud squeal coming from under the hood after the engine is warmed. It occurs at all times while in park regardless if the gas pedal is pressed. While in drive, it only occurs when in idle and after the gas pedal is released. Once I press the gas while in drive it goes away. I have changed the A/C compressor, tensioner pulley, idler pulley, alternator, and harmonic balancer, and serpentine belt. But the squeal is still present. I have removed the belt and started the engine, and with not else spinning but the motor, the squeal was this there. Any thoughts?

 

[98trck]

what year truck?
which engine and trans?
how many miles on the truck?