Engine humming noise

[Charles Ng]

I have a ML 55. Whenever I take the key out of the ignition, I can hear a humming noise from the engine bay. It will go away in a couple of minutes. There is no turbo in the ML 55. What is this pumping noise about?

 

[MotardMan]

I wonder if it may be a pump that circulates either coolant, oil or refridgerant for the A/C for a time aftyer the motor is killed. This feature is not unique to turbo cars. I had an old Renault engined Volvo that did this.