That strange MF noise
It's the focus motor. Manual focus is not directly coupled to the lens, it's fly-by-wire. When you turn the focus ring, it sends signals to the camera, which when sends signals to the focus motor to move the lens elements.
Which is insane if you think about it (versus the simplicity of mechanical focus). Why companies push focus by wire is beyond me.
It's cheaper and simpler to construct. Fly by wire requires a fraction of the precision a good focus helicoid does, and it lets you keep everything proprietary, mostly ensuring that your lenses have to be used on your cameras.
That's the skinny.