Hummmm....hummmmmm....

For several months now, (not sure when it started but also thought that I might have been hearing things, too, when I heard it), Collin hums to himself when he is thinking or relaxed. The sound is more like a light grunt or could be considered a quick hum.  I asked him why he did it and he does not know he is doing it...or does not want to talk about it.  Most of the time, it does not bother me.  However, it can be distracting when  I am reading to him before bed or if it's quite because it's a consistent noise.  Dad had not noticed this until we were in the car on the way to visit cousins and friends up North.  Collin was doing it while watching a movie.  I knew the part of the movie he was watching and knew there was not music....so this must be a habit or a "tick" that he does not know he is doing.