I do and sometimes I do it when I am happy, when I have had a good meal, when a friend is on the phone and they went to do something - it's my musical interlude. I think some people hum without noticing it. I also used to hum to my brother, it was our way of communiting when we were young and a game we played where we would hum a tune and then each of us would try to give a verse. Of course, this would get on my father's last nerves on more than one occasion.
I have a friend who hums to herself, but she is never aware of it until someone points it out. Maybe we hum as a sense of calming ourselves.
Any other hummers here? When do you hum and what types of tunes do you normally hum?