thats true and it happens when children arent ADHD aswell I know someone who does it if he feels uncomfortable about a subject others are talking about, he starts humming quietly ,he has no disorder, in fact he is a computer whizz.....
My eight year old hums to himself all the time. His doctor says it is because he has ADHD and it helps him tune out other things around him that might distract him.
It sounds like a tic of some sort. It's a habit that could stem from anxiety issues. You said that he is excessively shy around adults. Do you know when it started? Think back to when it started and see if you can remember what the situation was like. Have you tried to take him a child psychologist or your family doctor? They should be able to help you to see if there is something that needs to be done. It seriously sounds like a anxiety issue. I hope this helps. Good Luck.
I have seen this in children usually if they feel uncomfortable they can do it if someone is arguing it is a way of literally tuning out. I bet it happens when others ,maybe adults are having a' busy' conversation, also if he is occupied, I dont believe its of any concern I havent seen anything come of it. Maybe they grow out of it but I have seen a couple of adults do it....