It's excess vacuum built up in the engine after shutdown. What ever electric circuits read and control the electronics wastegate actuators are still functional and the system is attempting to reach some desired vacuum level. If you cycle the locks, or open the door this interrupts that electrical cycle and the noise should stop. You can also stay in your truck after shutdown wait for the noise and then press your brake pedal: it will stop.
This occurs not just on the new raptor but many boosted cars with electric actuators.
Here it is on the Mini Cooper
https://www.northamericanmotoring.c...ing-whirring-whine-moo-when-powering-off.html
And a powerstroke
https://m.youtube.com/watch?v=Gv-qLl9Urfk
Ford use to fix this, but has since come out with a tsb for the powerstroke saying it's normal.
FYI, it is normal unless it continues for periods greater than like 2 mins. This indicates a bad actuator, it will also be apparent when driving as your wastegate will not be functioning correctly.