Are you in a semi or a terraced? One house I lived in I thought I had an intruder but, when I jumped out of bed it was the neighbours the other side of the wall. Out joists were joined or something so when they walked on their floorboards it sounded as if they were in our house.
If you are in a detached then I have no idea.

Years ago in a rented house I would have sworn to you that someone was in my loft. It proper scared me to the point I couldn't sleep at night for fear my house was either haunted or an intruder had gotten into my home. DH found some rat droppings in the loft and set a few traps and as it turned out we had an infestation They were in the walls and chewing the electrics and all the movement sounded like footsteps or creeks. We never actually saw any evidence of rats inside the house and apparently they were getting into the loft via a large tree that was near to the house (apparently they can climb walls too! ) A friend had a similar incident with squirrels in her loft and I've also heard of bats getting into lofts. Needless to say we moved soon after that. Not sure what is worse ghosts, intruders or a rat infestation??

I recently remembered that on one of the first nights we moved to this house, as I was drifting off to sleep I heard what sounded distinctly like footsteps in the attic above. Never heard it since - i suspect it I was just half asleep and it was unconscious anxiety or whatever about moving into a new house!