I've recently found this article: www.spr.ac.uk/main/news/colvin-acoustic-properties-poltergeist-rapping
Needless to say I find it less than convincing: the sounds were not examined in situ but on records of variable quality often dating from decades before (Enfield Poltergeist).
This very different from the so called Berry-Kirlin-Hertel case (alleged Bigfoot vocalization), where noises were taped in situ and immediately dispatched to a research laboratory together with the original taping equipment used.
Also there's always the big question: is the record genuine? Was it really a rapping during a poltergeist case and not, say, a completely unrelated noise?
"Louhi spoke in riddled tones of three things to achieve: find and catch the Devil's Moose and bring it here to me. Seize the Stallion born of Fire, harness the Golden Horse. He captured and bound the Moose, he tamed the Golden Horse. Still there remained one final task: hunt for the Bird from the Stream of Death"
-Kalevala, Rune XIII-