My brother is 18 and here’s some back story. 
Ever since he was a kid he would sit in a corner facing the wall with his knees to his chest making explosion sounds and rubbing his hands together imagining things. 
As he grew up he didn’t do it in public and we thought he stopped but I did and still often catch from the they room the the sound of him making explosions. 
Still now, he’s 18 and I still hear him doing it. I just hear gibberish and explosion sounds like he’s imagining a story in his head. He has no other problems. He acts completely normally. He is very stubborn, kind of irritable, and critical. Does he have some kind of disorder or is he just imaginative?