[OMFG!You'reActuallySmart!]

Worrying about the sounds adds in a psychological element that it's simply your mind casting the perception of a ghost. The sounds may indeed be real but working yourself up to think something/one is there, such as a ghost, over and over will eventually make you believe just that regardless of how false it actually is. For this reason, I don't think a voice recorder is going to do any good because if it detects a sound, then you're back where you started anyways: trying to figure out the cause of the sound and the tape recorder cannot help in this problem.

So in order to figure out the cause, which may simply be the house settling, a branch hitting the window, critters running on the roof, etc..., you need to get the psychological element sorted out, otherwise you'll continue to believe it is ghosts even if you're wrong. Inviting friends over may be useful if they can help with calmness, otherwise they'll only add to the chaos and stress, and that is of no help to you.