DD has a habit or repetitively humming, or mmore like droning, when she is concentrating on something like drawing, or doing something humdrum like the walk back from school, or when she is tired. She has done this since she was very tiny and she is now nearly 5. It is often very loud. It's clearly some kind of zoning out technique.
I'm not worried about it as I know she can stop it when she is at school (the teacher has asked). I am more interested to know why she does it, and if anyone else's DC does it and will it ever stop!