DS sings a lot very audibly while focusing - actually, the humming is more than the singing. It was a problem in 1st grade, so he stopped doing it in class (very strict teacher and he was probably scared of her or wanted to be in her good books). The 2nd grade teacher is OK with it, so he is humming or singing at school now while he works. At home, when he is alone and is focusing on work, he sings. Music is a part of him - he is a very musical child and he is always humming a piano piece he is working on, a tune he heard somewhere or an original musical composition. The musical compositions are always for a soundtrack of an imaginary episode of the story of the bad guys vs good guys that his brain is constantly visualizing.
The K and 1st teachers had a problem with it because it distracted the neighboring kids from their work. I guess his singing and humming is quieter now because it does not seem to cause any distraction any more.
