From the 2005 Expedition Owner's Guide...
REVERSE SENSING SYSTEM (RSS)
The RSS sounds a tone to warn the driver of obstacles near the rear bumper when R (Reverse) is selected. The RSS will assist the driver in detecting certain objects while:
• the vehicle is moving toward a stationary object at a speed of 3 mph(5 km/h) or less.
• the vehicle is in R (Reverse) but not moving backward (the brake pedal is depressed or the parking brake is applied), and a moving object is approaching the rear of the vehicle at a speed of 3 mph (5km/h) or less.
• the vehicle is moving in reverse at a speed of less than 3 mph (5km/h) and a moving object is approaching the rear of the vehicle at a speed of less than 3 mph (5 km/h).
The RSS is not effective at speeds greater than 3 mph (5 km/h) and may not detect certain angular or moving objects.
From the 2005 Expedition Workshop Manual...
Parking Aid - Principles of Operation
NOTE: The liftgate must be closed for correct operation of the parking aid system.
The parking aid system detects objects behind the vehicle when the vehicle is in REVERSE. The parking aid module does this by calculating the distance to an object by the use of 4 ultrasonic sensors mounted in the rear bumper. The ultrasonic field is emitted in a semicircular area around the rear of the vehicle. The parking aid sensors can detect objects approximately 1.8 m (5.9 ft) to the rear of the vehicle, 46 cm (18 in) to the rear side of the vehicle, and 16 cm (6 in) above ground. A 750 Hz variable warning tone is generated from a parking aid speaker attached to the parking aid module. The parking aid speaker increases its warning tone rate as the vehicle gets closer to an obstacle. When an object is detected within 25 cm (10 in) of the sensors, the warning tone becomes continuous.
The parking aid system is enabled when the ignition switch is in the RUN position and REVERSE is selected. The system can be disabled through the message center RESET switch located in the instrument cluster. The parking aid menu is displayed until the transmission is taken out of REVERSE. While the menu is displayed, pressing the message center RESET switch changes the parking aid system status to OFF.
The parking aid system is disabled if a fault is detected in 1 of the 4 sensors, the parking aid speaker, or the parking aid module. This will be indicated by the parking aid disabled warning CHECK BACKUP AID displayed in the message center.
Good luck!