Self-stim

He self-stims when I'm not watching, and sometimes even when I am.

Source: wiktionary

Perhaps that is why your child is rocking back and forth, tapping his finger, or whatever he does![…]Or your child could be sensation seeking. That is, he self-stims because it feels good!

Source: wiktionary

"Looks like Mommy really screwed up this time," she said to the nonverbal child who looked only briefly at her then wiggled to the floor where he rocked back and forth while flipping his eyes left and right, a move he could do for hours and which the doctors called a "self-stim". They said it was common with kids with Angelman Syndrome.

Source: wiktionary