For a reason that I no longer recall, I turned the ignition switch to the On position in my 1997 XK8. The hood happened to be up at the time and I noticed a humming sound, like an electric motor or pump, coming from the throttle body. The noise is transmitting through the assembly so I can't determine which of the components is causing the sound.
Is this something I should be concerned about? I can't imagine that the fuel pump is located in here so I don't know what could be causing this sound.

 

It’s most likely the coming from the motor of the throttle body, nothing to worry about. Just press the gas pedal, and you will hear the sound slightly change to confirm it’s from the TB motor.

 

I've got the same sound coming from my throttle body, can be heard when the ignition is "on" but the engine isn't running. Kind of a "whirring" sound, or a high-pitched hum. Hasn't evolved into any other type of problem.

 

Although the AJ26 Throttle Body looks similar to the AJ27 Body, they operate very differently.

The AJ26 Throttle Shaft is spring loaded to OPEN without Motor operating. The Accelerator Pedal being released or the Motor activation keep the Throttle Closed.

The AJ27 Throttle Shaft is spring loaded to CLOSE without the Motor operating. The Accelerator Pedal being pressed and/or the Motor activation opens the Throttle.

For the XK8, the AJ26 is used in the 97 & 98 MY. 99 - 02 use the AJ27.

For the XKR, the AJ26 is used in the 97 - 99 MY. 00 - 02 use the AJ27.

 

Thanks everyone for the information and reassurance.

 

Almost all fuel injected cars have some sort of idle bypass valve. Its purpose is to fine tune the idle of the engine, compensating for temperature, altitude, cold engine, etc. It is activated by putting the key to the run position, but for some cars, its active in the on position as well.

The sound you hear is the duty cycle. Some bypass valves use a electromagnetic coil to move a plunger or door. The coil turns on and off (duty cycle) in rapid succession to position the plunger or door to the optimum setting. Because the plunger or door is spring loaded, if the coil is on longer than it is off, the plunger or door will open more than if the coil is off longer than on. The duty cycle (time on compared to time off) controls the position of the plunger or door.

Some bypass valves use a motor to move the plunger or door. They also have a duty cycle, but this is a forward/reverse situation, instead of an on/off action. The ecu of the car continually checks where the plunger or door should be, and drives the motor forward or reverse to set that position. The position is continually being adjusted, so you hear the motor running forward and backward non stop.

Because the duty cycle is so short, and the ecu adjusts the plunger door so quickly, you hear a high-pitched whine or hissing sound.


In one sentence: This is normal.