I've developed a habit of humming, whistling, or singing along with whatever music I'm listening to, usually classical music. This isn't news to anyone close to me, as I've been doing this for years, probably as far back as my teens. It's sort of like the noises the late Glenn Gould made on his recordings. I'm aware of this habit, and when I'm at work or in a quiet environment I try to keep it down. But when I'm walking or, more recently, on the subway, the occasional odd hum or scat-like sound comes out. This evening I was waiting in line at the grocery store and listening to a Beethoven piano concerto, and absent-mindedly humming along with the music. The guy in front of me must have heard me, because he turned and looked at me like I was crazy. I'm not really self-conscious about this habit, but I'm beginning to think I should be.
The good thing is that unlike Gould, I don't hum or sing along when I'm playing the viola. I can't play and talk, sing, or hum at the same time. In fact, when I was in college I had an exam for a music theory class where I had to play one melody and sing a different tune at the same time. It was one of the most difficult musical things I've had to learn, and I doubt I could do it again now. Also, this habit is not to be confused with my other habit of "air-conducting" along with classical music. Since I live alone and sit in a cubicle by myself at work, I'm not at all worried that someone is going to see me conducting an orchestra that isn't there.
All of these weird idiosyncracies are probably signs that I should have pursued a career in music, or at least gotten back into active playing well before this fall.