If you shut your radio off and the car is still on, there is no signal controlling your servo. It will move. If you do that at a track, it will move more than an 1/8th of a turn.

The humming is from loads being applied to the servo. All it needs is for you to put a little side load on the wheels and it will hum because its trying to stay where your Tx is telling it to stay. Its all normal servo functions. TTYL.

Gary