My friend used this Sleep Recorder app last night and it registers when noises around are made when you're (obviously) sleeping and nothing else. She got it because she lives in an old house and is paranoid she's had a ghost for years.
At like 5am it recorded three loud bangs when everyone was sleeping (it only displays when noises are heard so you don't have to sit through 7 hours of recording) and it kinda freaked me out so I'm gonna use it tonight and see what it picks up.
Are you brave enough to try it? You can get it here.
You can get it 
