Okay first, let's not jump to ghosts and ghouls right away. It could be something as simple as the house settling or pipes expanding and contracting. Have you checked for any drafts or structural issues in the attic? An old house can make all sorts of noises.

I'm all for a spooky story, but this really sounds like an old house problem. Houses settle, and temperature changes can make weird noises. Maybe check for rodents? Could be they're the ones having a party in your walls.