My uncle is an ex policeman. He tells the story of an old lady who complained that she was being woken up every night by loud knocking on her front door. My uncle kept an eye on the house and found no reason to suspect anyone of disturbing her. A chance meeting with her doctor led to the discovery that she had a mild irregularity in her heartbeat. She was woken up by a knocking sound, which in actual fact was her own heartbeat.