I've been considering this for a while, and I've reached the conclusion that the songs on the Harry Potter 3 (Prisoner of Azkaban) soundtrack may have become the default music that plays in my head when I'm not actively thinking. (Particularly, that "Double, Double, Toil and Trouble" song and the Knight Bus theme.) Other songs get stuck in my head intermittently, but these seem to be permanent. And they don't ever annoy me - they're just there, floating around/behind/in lieu of my thoughts.
Am I the only one who has experienced something like this? What is your brain's default music?
ALSO, if your brain lacks go-to songs to play over and over, tell us about that too. (What do you think (or un-think) about? Is there something non-musical that loops around your mind? Do you just hate music?)