How long does it overrun, seconds, minutes?
There are several normal sounds that you will hear on today's electronically controlled automotive systems. Fuel pump, fuel canister purge, fan, AC/Heater motor, etc. I suspect it's one of these systems.
Personally, I believe it's the electric coolant pump to cool the turbo after shut down, circulating coolant while static cools the bearing cassette to help prevent damage and oil burning (remember no oil circulates while the engine is off but the turbo still spins.) Hyundai has been using this system since they went to the turbocharged gas engines. This is why it's no longer necessary to idle turbocharged cars, after you finish driving to let the car cool down.