My 6 year-old grandson hums and sings most of the time. This started about 5 months ago. He acts and seems very happy. But his 12 year-old step brother has now moved in. They get along, but not as loving as I would like. I think he has some jealousy of the 12 year-old. I'm worried that now that he is a middle child he may be having issues over this, and that the singing and humming is a defense action.
Perhaps, it's his way of not drawing attention whilst wanting attention. A parent is more likely to give attention to the child who is talking or playing rather that the child that is humming to themselves and seem content. When participating in a blended family a parent is probably trying to make sure everyone is happy therefore focuses on the child who seems the most distressed. Additionally I find that when I feel pushed aside I do something to make that seem justified, do something that doesn't draw attention or is annoying so that when people don't pay me any attention it's because I don't want them to rather than they don't want to. If your child is silent you talk to them, if they're doing something you talk to them however if they seem content, you leave them be. He may be worried about his place in the new family, so he doesn't want to get to involved so he doesn't have to find out where it is, then if it's low down, it's not as bothersome.