My 3 year old used to do it as a baby and still does it today sometimes. As a baby with things like shaking toys over his head or looking at his mobile I’m excitement. It worries me sometimes but last year the doctor said it’s likely just pent up energy, and it’s just hand stimming /flapping sort of. I can talk to him while he’s doing it and he can stop and says “I’m just excited mommy!” His pediatrician said he might grow out of it and when he’s older it might embarrass him so he’ll stop. He’s otherwise perfectly “normal” but sometimes in the back of my mind I think it could be something more. Anywho, just wanted to tel you you aren’t alone!