I've had the same thing for as long as I can remember, long enough that I can automatically tune it out. What's funny though, is sometimes I will hear music playing in my dream and when I wake up the music is still playing (and often remains there for a large part of the day). In my case it's partly related to my GAD; anxiety makes my mind loop and keep replaying things as an automatic self-soothing thing, and finding a good medication for me helped. I think it is partly hallucanatory; when I was taking seroquel the "music in my head" really slowed down and got quiet and wasn't carrying over between my dreams and waking life. Now that I'm not on seroquel it's picked back up...