The grinding sound is the hydraulic PTU self-testing when the second engine starts. It usually only runs when there's low pressure from one of the engine pumps. There is an electric hydraulic pump that runs all the time when the engines are running, but it's not particularly loud, and you won't hear it start up usually. The yellow pump you turn on for towing is pretty noticeable from the cabin, but it is typically not turned on for flight. And, though the flight controls are controlled by computer, they are actuated hydraulically. Hydraulic power is essential to aircraft control, so your statement that it's not needed in flight isn't entirely accurate. Just thought you'd want to know.
As far as that banging sound is concerned, it's probably the sound of the brakes being released. There are huge drum brakes on the gear, and they can make a racket.