Bang Bang
If you’ve watched “The Mummy” film series, you will agree that nothing good can come out of an ancient box. The OP’s father-in-law found this box under a house and from the looks of it, it seems to be from a long time ago. It also appears to be the kind that can’t be destroyed easily.
The scariest part, however, is that the OP knows there’s something in there because they can hear it bang. Wait, what? An antique box with an object that moves? We’ve seen enough horror movies to know that box should never be opened. In fact, we’d feel much better if someone would just throw it in the ocean.