Funny - this was my youngest at that age, except he would also hum songs he was composing in his head. He would sing so loud in Costco that people several aisles away could hear him. He hummed in the car, at home, in bed at night. It never bothered me, though - I liked hearing the sounds of a happy child.
When it became a distraction in class - especially after he roused the class with a rendition of Canon in D where the class ended up marching around the room with all of them humming, his kindergarten teachers asked if I would work with him to help him find a way to stop humming during tests or quiet time.
I asked him why he hummed, and he said that the notes were so loud in his head, he just had to hum them to hear what they sounded like out loud. He said he wasn't always aware that the humming was out loud.
We finally worked out a system where the teacher would just gently touch his shoulder when he started to hum at an inappropriate time. It was enough to break his concentration and make him aware that the humming was out loud.
He's ten now and plays his keyboard with the headphones in, sometimes for hours. He hums around the house and at school but has learned to squelch it when it's a distraction.
Hope this helps.
