The Ford Expedition is known for idling roughly, with a slight whistling noise from the engine compartment. After a short time, this may result in poor fuel mileage, and illumination of the check engine light.
This is caused by a cracked positive crankcase ventilation (PCV) valve hose, which allows a small vacuum leak, and improper evacuation of engine crankcase pressure.
Correction is simple, and requires replacement of the PCV hose. An updated part may be available.