I found that rhythm in typing, too. I first started writing on a typewriter, and that rhythm was satisfying.
After finding it in writing in notebooks and on computers, I realized that it’s the rhythm of the writing process – thinking, putting it down on something, editing – that I enjoy. The typewriter’s clacking, thumping, and bells (whether on my old Brother portable or my second-hand IBM Selectric II, which sounded more like a machine-gun firing out letters when I was going at speed), the noise of pen on paper, the computer keyboard clicking, is all melody to the writing in my head.
The movement and flow of writing is addictive and satisfying like nothing else the world offers.