Wait, that's a problem?
I've done this since before I could remember. My #1 does it constantly. We have had to discuss how it affects other people, but I don't see a need to extinguish the behavior.
As a child, I remember that I definitely did it because I was happy, and also to focus myself. I used to sing my study materials, too. In college. Made me a great study group partner--not!
If this is a problem at school, I'm not sure I would consider her quiet humming to be the issue, so much as the classroom culture. Most first grade teachers I've known don't mind soft humming or singing in children who are on task.
