What Causes The Child To Rock Continuously?

How can you get a 11 year old to stop rocking? He only does it when he is watching t,v, or is in the car. He stops if I tell him, but he does it with out even realizing it. He play well .. interacts well. play sports, and is an high honor student so I don t think he is autistic but I wonder about the rocking and noise bothering him