door open electric motor sound

brake accumulator. it also tops up when you're pressing the brake from time to time.

 

What Bisco said. Prii love to be driven. It's like your dog fetching the leash when you put on your shoes.

 

Normal cars have a belt driven hydraulic pump driven off the crankshaft. The Prius is a hybrid that often runs on purely electric power where the engine is not running, so the brake accumulator is pressurized with an electric motor/pump.

As mentioned, when you open the door, the electric pump is pressuring the brake accumulator so you have brake function when you make the car Ready and drive off.

 

The very first thing you hear (if you unlock the doors), is the tiny little electric motors moving the door locks.
But the other answers are probably what you are really asking about.