Use a stern voice, and politely tell it to bugger off. It's probably angry that you are a) not related to the original owners and b) not acknowledging it. You have to tell it that this is your house now and that it has no right to be there. It doesn't seem malicious towards you two, but the increased activity might be because of the kid. So, when the kid is born, it may try to mess with it. Try telling it that the child has more right to be there than it does. Also, not sure about this, but try putting salt each corner of the baby's room (going anticlockwise). It's meant to stop ghosts from entering rooms.

An old house that was past down by family will usually have some haughtings, but know that you have more power and authority.