There's a whirring sound when I shut off the engine. Is this normal?

Hello, Yes, this is normal, and, indeed, it is an electric motor winding down. You are likely just hearing the cooling fans, as the vehicle will need to run them after the engine is off at times. Pop the hood, turn off the truck and watch the fans. You can perceive the speed of the fan based on the sound, and if fan speed and sound do not match, there is something else going on. Possibly a secondary electric water pump.