This banging could be anything, especially if it's an older house. It could be water or other pipes or a breeze blowing something around that isn't secured well enough.
Put a recorder, tape and video if you can, downstairs and see if you can capture a few EVP's going on (voices that you can't hear normally) and maybe catch a shadow or movement on the video. Of course, everything is better in digital, and for the EVP's an actual 'tape' may not record long enough to catch anything. And you should leave on a very dim light for the camera unless you have a night vision one.
You may want to take a few pics down there too to see if you can catch an orb or two, best chance is when the noise is going on.
You should look into it first and make sure nothing is loose that can be blown around. Nothing hanging down, like clothes, that can be mistaken for the unnatural. Make sure things that could move and impact other objects are secure and can't move.
Set up the camera to watch where you think the banging is coming from, and set the audio recorder near but not in the same area, let them run all night and see what you have in the morning.
EMF meters are good, but be sure to take a base line reading before the actually looking for a change in the numbers. Electrical wires and boxes are notorious for drastic number changes. Light sockets will change the numbers. Some piping material will also change the numbers on the meter, so be sure to get the base line numbers first.
I'm betting that it's a natural thing and not a haunting.