I think some kids just like to hum. My ds8 hums all the time,and,yes, it's now Christmas songs. We joke about it (it's in our Christmas letter this year as one of his hobbies). Most of the time the teachers are okay with it because it's pretty quiet but we hear about it EVERY year. In fact, we usually warn them in advance.
So, no good solutions, but some empathy. And it's weird, ds10 almost never hums. Dds20 and 18 were both singers (dd18 was in the country's top show choir and it consumed her) and yet, they didn't really hum either.
Good luck
Theresa
