i hear music sometimes, and dings or dongs, amongst other background sounds. never really concerned me too much. i think the brain just tries to make structure out of random strings of sounds. like how when we look at objects and find faces in them.
Like this:

Many people see a "face" in this satellite image of the Cydonia region on Mars, due to the human ability to perceive faces even in inanimate objects, a phenomenon known as pareidolia