I believe I have bearings going out in one of the pulleys that the serpentine belt rides on. It sounds like a bearing is squealing when you let off the gas pedal, just before the engine is back at idle speed.
This is a common issue on the Expedition. There is a tensioner pulley, and an idler pulley. To verify which of these is the cause of the issue, an engine stethoscope can be used to listen with the engine idling, or the belt can be removed, and the pulleys allowed to spin individually, to determine which pulley has the bearing issue. You may want to enlist the help of a mechanic, such as one from YourMechanic, to assist in the location and replacement of the faulty pulley.
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