Mechanical hum/whine when engine turns off

[Horrgakx]

Car is C250 AMG Line Premium Plus Diesel. When I turn the engine off I notice a faint mechanical hum. Does anyone know what this is? I suspect that it could be an electric pump which circulates engine oil in order that the turbo oil doesn't cook. But does anyone know for sure?

Link to the sound is below. It's the constant sound I'm talking about, not the engine pinging as it's cooling down...
https://soundcloud.com/dave-wilson-24/mercedes-hum-when-engine-is-off

 

[BTB 500]

It's not a cooling fan for electronics? This runs for a minute or two after switching off our Vito.

 

[Horrgakx]

It's not a cooling fan for electronics? This runs for a minute or two after switching off our Vito.

Trouble is I can't tell where it's coming from, near the engine somewhere (I recorded that with my iPhone placed between the engine and the radiator at the front).

I just love to know what goes on 'under the skin'. There are so many things 'active' even when the engine is off.