What are the possible diagnoses with symptoms of rocking and humming with hyperfocusing?

I have scheduled a doctors appointment for my 8 year old son already. I am fully expecting him to be referred to a psychologist, however I would like to be fully prepared for what I might hear. I have found all kinds of information on autism, but my son does not have any other symptoms ociated with autism (only humming and rocking). He is very social, he is the popular kid in cl. He gets good grades without having to study much, but does have some trouble spots. He is eager to please others, he is very sensitive to other peoples feelings and does what he can to help. He is an extremely normal kid, except for this humming and rocking, but he only does it when he is really focused on school work. When asked about it (which was done very cautiously as I did not want to give him the impression that something is wrong with him) he says it helps him concentrate. His teacher and I have talked a great deal about this and together we have (w/out MDs) ruled out both autism and AD/HD because of a major lack of symptoms. Does anyone have any others thoughts about what we might be dealing with? (The reason for concern is the distraction to other students) Thank you for your potential help!!