Gosh, how frightening. Can you contact the previous house owners, and ask if they had any experiences of this?
Trying to be logical, have you a roof space? I ask this because, sometimes, birds esp large ones, like seagulls or crows, can sound very noisey running across a hollow floor, or rats?
On the other side of things, with no logical explanation, I would contact the previous residents, and also a priest, and failing that, a medium (as some people do believe in them, I'm not too sure about my thoughts on that particilar thing).
Keep us update.