Where do all the random thoughts and impressions come from?

What does Buddhism say about the random stuff (sankhara?) that just comes into mind when I am meditating? And I mean random, like I'll be meditating and a memory just comes out of nowhere, sometimes a half-visual, half-felt scene of a place I travelled 20+ years ago. Or I'll just start thinking of a person I haven't seen for a long while. This is while meditating, so there is no sense input ~ it's not like I heard a song that reminded me of someplace. It's like my mind has a mind of its own! Is this kind of material related to the "storehouse consciousness"? I think of it as like sitting on a cauldron, because this material just continuously bubbles up ~ sometimes it is an angry boil, sometimes a gentle roil. I would love to learn more about what Buddhism has to say about this phenomenon.