I agree with the suggestions that it's the central heating or the plumbing. Is there a radiator near the corner that the sound comes fromn ? - do you get the noise just after the system has kicked in ? Our heating system definitely produces expansion noises in the morning and on of them is a sort of knocking as floor or skirting boards expand. It could also be air in pipes as has also been suggested - you could try bleeding the radiators (should be done a couple of times a year anyway) to see if that makes any difference.

Why not lift up the floorboards and have a look - mind you just to be on the safe side make sure that you've got a crucifix, some garlic, a pistol with a silver bullet, a wooden stake and hammer . . . . . .