I didn't much notice when ds (then 8) stopped humming and singing around the house, just a vague notion that he'd lost interest in singing. However, turns out he was depressed. Now that he is feeling better in general, he is humming and singing again most of the time, and I am so pleased. When he stops, I will worry.
I tried to google around for articles on humming and mood, and that sort of thing, I was surprised there aren't more as this seems like a great indicator. (someone should do a study!!)