When the car is turned on but engine is not started, there’s a humming sound coming from the engine compartment. What is it? I suspect it should be related to electricity but not sure.

Fo unknown reason, when I sit in the car hearing this humming sound, my left ear would feel continuous pulse like the heart beating but not sync with my heart rhythm, this totally goes away when I shut off the car and the humming sound stops.

Any one can relate?