Maybe its because of the fuel pump ballast resistor.
A ballast resistor is installed in-line between the relay and the fuel pump.
The Engine Controller operates the fuel pump through the fuel pump relay during all operating conditions. This will be except for wide open throttle (WOT) and high engine loads. The ballast resistor reduces voltage to the fuel pump, reducing the level of fuel pump noise.
When the ballast bypass relay is energized, voltage is applied to the fuel pump through the ballast resistor. During WOT and high load operation, the Ballast Resistor Relay is de-energized and the ballast resistor is bypassed
The ballast bypass relay is located on a bracket connected to the Power Distribution Center.
Voltage is supplied to the fuel pump through the ballast bypass relay during heavy load or wide open throttle (WOT) conditions.
Voltage is supplied to the fuel pump through the ballast resistor, except during heavy load or wide open throttle (WOT) conditions. The ballast bypass relay is controlled by the Engine Controller.
The Engine Controller energizes the ballast bypass relay by switching the ground circuit to the relay (on-and-off). Battery voltage for the bypass relay is supplied through the fuel pump relay.