Do you find yourself with tunes running around in your head?

I noticed recently that every time I go walking, there seems to be a tune running through my mind—almost like a marching song. Sometimes it’s a well-known tune (yesterday the Hannah-Barbera theme was there), but usually it’s just something that I don’t recognize.

I don’t think up these tunes, or at least, I am not aware of any thinking that goes into them. They just appear, like magic. When I become aware of them, I tend to get bored of them, because they just repeat themselves over and over, unless I do something about it. So, in a more conscious way, I’ll start changing them, and improvising off them.

Does this happen to anyone else? What do you make of it? Where do these tunes come from? What do they mean?