Wow really neat that not only can your friend verify that there is a spirit present, and he/she can tell their power level as well. Pretty cool for sure.
My first question is 2 part - a) do you REALLY want to find out what it is?

do you want it to stop?
If you are serious about either of those, then it will cost you roughly $75 to $125 to identify and rid yourself of the problem. For the $75 you can get a very decent 3 camera+dvr security system with all the cables needed to set-up. For abit more you can add microphones to those cameras, which sounds like your case. You might find some for less than teh $125, but I shot high.
Neither of these requires any technical skills or permanent fixtures to set up. The result will be one of the following:
1 - sound stops, nothing appears, problem goes away but you never know who/what was knocking...meh oh well, peace achieved.
2 - the cameras spot someone maybe person or creature (not cryptid hehe think dog etc) messing around your door.
3 - You capture the first ever tangible audio+corresponding video evidence of the paranormal - you are world famous, on Letterman, subject of a made for TV movie, science is at your beck and call to investigate and provide help.
So most of those sound worth the $75 t0 $100 bucks, your call.