My DS does this while drawing, while running hands under water and actually used to make the same sound as an infant when trying to fall asleep in the Ergo!
I think it is the cutest thing ever but it does draw concerned glances from folks who don't know him.
(He is only 19 months but I'm posting here because I was ID'd as a gifted child and my little peapod has a bunch of early signs, including the fact that he is talking like an average 3 year old: 5-10 word sentences, over 1000 word vocab, using correct verb tenses, possessives, adjectives, adverbs and lots of conceptual words)
DD sucks on her lips when she's concentrating, DS and I both making a quiet humming noise, and DH does the traditional stick out the tongue thing.
If it looks like I'm trying to pick a fight... I'm not, I'm rarely that obvious.
Mountain mama to two great kids and two great grown-ups
One of my DD does this clicking noise she makes(and has since infancy) when she is thinking/concentrating.
My other DD sticks her tongue out of the corner of her mouth when doing something challenging!
Just individual quirks--- I am sure a lot of people have them.
Does he flap his hands or make weird hand gestures,or clapping? My friend has a son (5 yr old) who makes grunting/humming noises and also claps his hands when he gets excited. Her son has a disorder called "flapping" that is associated with autism but non-autistic kids sometimes have it too. Otherwise he is totally normal. I've known his since he was an infant and I just thought this was his mannerism but there is a little more to it than that. If your son does flap (the hands) you might research it a bit. As with most things it can be mild to severe and involve other symptoms or stand alone. My friend just touches her sons hands when he does the behavior to make him aware or if he makes the sounds in public she might ask him to be quieter- she doesn't want to make him insecure about it but they get looks in public so she wants to help him learn to control it. She lets him flap and make noise all he wants at home.