What's that noise?

Jack makes me laugh all the time. He was just riding his bike in the loungeroom and making a funny noise with his mouth. Sounded high pitched and loud, like "oooooooooo!" He stopped. Looked at me and said "what's that noise?" with a frown on his face. I said "it's you making that noise" and he went "oh. yes" laughed and kept riding. So funny!