There's always some music running through my head - I was quite old before I realized that most people's brains don't provide a running soundtrack to life, hence the popularity of the iPod!
But sometimes a song gets stuck there, whether I like that particular song or not!
Right now, "Jar of Hearts" is stuck in my head, by Christina Perri. It's a pretty melody, plaintive, and not as angry as the lyrics would imply. What's stuck in yours?
And please, do tell me whether or not your brain does the 'background music for life" thing, I'll be interested at how common it is.