Taken from the Society for Psychical Research website. Copyright respected.
Paranormal rappings associated with apparent poltergeist activity have been described for many hundreds of years. It is only now that an interesting pattern has been discovered within the fine detail of the paranormal rapping sounds. No explanation can be found for this pattern at present.
The current edition of the Journal of the Society for Psychical Research (SPR), a learned publication dating back to 1882, carries an article by scientist Dr Barrie Colvin B.Sc., Ph.D., showing instrumental evidence for an inexplicable and objective banging sound detected in recordings made during alleged poltergeist activity.
Whereas raps and knocking sounds produced by ordinary means exhibit a normal acoustic pattern, those recorded in alleged poltergeist cases show quite a different sound signature.