I'm sick of seeing the response by some to either replace the humming with a new repetitive behavior or just accept your humming.
That's no solution!
I suffer from this to a point that it causes great distress, embarrassment, humiliation and interferes with my functioning even worse... amplifying my anxiety that the behavior originated as a way to calm my anxiety!
In the beginning I had only one repetitive behavior and now I have layered simultaneous embarrassing repetitive behaviors... humming, sighing. rocking, hand and teeth clenching... I tried many self-soothing behaviors and they didn't REPLACE anything. Things have become far worse, more frequent, for longer episodes and more embarrassing!
This is something to ask your doctor about. I can tell you that children that have a nervous system that is not regulated sometimes hum or make noises. My son has sensory integration disorder and this question was asked several times during our evaluations.
some kids may do it to tune out as margypops say and some do it to stay alert.
They sometimes offer an oral soothing item (straw cut in half to chew on works or they make some you can buy) or a hand fidget can help.
But run it by your doctor. good luck
Some people hum. I hum softly to myself much of the time. I also sing a lot - at work - at home. I don't think it has any special meaning except for an affinity for that kind of sound. We are all a little batty. It is called personality.
I have heard this a few times, my SIL still does it , he says its to tune out ...have you noticed it more when she gets upset?