Are there unexplained, dark stains that can't be washed out? At night, do you hear chains dragging? Are there blue mists in the halls at night that are cold when you walk through them? Do you hear groans that don't seem to have a specific source? When you arrive home after dark, are there lights in windows where lights shouldn't be -- and they go out as you approach the house? Are there lights in windows where there aren't any windows? Does Anne Boleyn drop around, her head underneath her arm?

Seriously, pinpoint the spot as closely as possible and then lightly touch it. See if you can feel vibrations. If you can, the problem would seem to be in the boards themselves (e.g., Death Watch beetle). If not, consider that the acoustics are lousy and that the noise is coming from elsewhere.