Here's a familiar story. Two young girls are in bed one night when they hear a curious tapping noise coming from somewhere in the room. This happens on several consecutive nights. It seems to emanate from the wall, and they think at first it must be coming from the house next door. But then, weirdly, they realize that the noise is responding to them, even when they are whispering so quietly that no one outside the room could possibly hear. They find they can communicate with it, by asking questions and getting it to knock once for yes, two for no, and three for don't know. For more complex queries it will rap out the letter of the alphabet (five knocks for E, 13 for M, etc). The whole family soon gets involved, and gather nightly to ask the unseen entity about itself and get it to answer questions about themselves, which it often does correctly. The house is soon filled with neighbours, local clergy, police, mediums and investigators, all coming to wonder at the phenomenon and try to figure out what's causing it.

The strange story of the Fox sisters is usually the first thing that you read about in any general book about spiritualism and the paranormal. You may go on to hear that that having established they could communicate through raps the spirits later came through at séances, launching the cult of spiritualism that quickly swept the developed world. If it's a debunking book the mystery will then be revealed: towards the end of their lives the girls admitted it was a prank played on their parents, first by bumping apples tied to string on the floor, and then by manipulating their toes and joints to create the rapping noises. This segues naturally into reflections about the gullibility of the superstitious masses, and their reprehensible failure to accept it was all a trick.

Either way, the impression most of these books leave you with is that the Fox incident was a one-off. But of course this tale of raps and codes and spooky communications is widely reported. It's not exactly common, but it's so distinctive, and often reported in such detail, as to create the appearance of a phenomenon in its own right. When Tony Cornell and Alan Gauld tabulated 500 documented poltergeist-type cases back in the late 1970s they found that around half involved exactly this kind of rapping noises, often described as knocks, thumps, thuds, bangings and suchlike, for which no cause can be found. They say 16% involve communication, of which presumably the majority involve this method. [Poltergeists, pp. 224-40]

The case I mentioned earlier is actually not the Fox sisters, but concerns the Andrews family in Andover, Hampshire, in 1974. It was investigated by Barrie G. Colvin, who says he was prevented by the family from publishing more than an outline at the time. Ten years later they were still unwilling to have it publicised but now that more than 30 years have elapsed, and the family has moved from the area, there is no longer an issue about this, and he has written it up in the latest SPR Journal, using pseudonyms.

Colvin seems to have been quite thorough, paying a total of nine visits over a ten-week period. As well as interviewing the family about the origins of the case he had plenty of opportunity to hear the raps himself and establish that they were not the result of trickery or other visible cause. The focus seems to have been Theresa, the younger of the two girls aged 12. Colvin also established to his own satisfaction that the source had intelligence of a sort, calling itself Eric Waters, although it does not seem to have provided any coherent information beyond that. At one point a medium claimed the noises were being made by a young boy whose body was buried under the floorboards; nothing more is mentioned about this, and subsequent investigations failed to turn up anyone of that name who had lived in the area.

Colvin did attempt a small experiment, persuading 'Eric' to transfer the noises from the wall of the room to the headboard of Theresa's bed. As follows:
[Mrs Andrews] then said: "Eric, please try to knock on the headboard." This was followed by a very soft tap which was heard by us all. I was at that moment standing very close indeed to the headboard, with my ear about 15 cm from it. As Mrs Andrews repeated the request, I put my hand on the headboard to see whether I could feel any sensation. Eric rapped progressively louder on the headboard and I could clearly feel the vibration.
It's interesting how often vibrations in the bed headboard feature in poltergeist literature. This is just one example, from the 1960 case in Sauchi in Scotland:
On entering at the front door he heard loud knockings in progress. Going upstairs he found Virginia awake, but not greatly excited, in the double bed... The loud knocking noise continued and appeared to emanate from the bed-head. Mr. Lund moved Virginia down in to the bed so that she could not strike or push the bed-head with her head, and he also verified that her feet were well tucked in under the bed-clothes, and held in by them. The knocking continued. During the knocking Mr. Lund held the bed-head. He felt it vibrating in unison with the noises. [A.R.G. Owen (1964) Can We Explain The Poltergeist?, pp. 148-9.]
The responsiveness is less common, but is still widely reported. Perhaps the best known case of the kind is reported by William Barrett, investigating a case in a farmhouse in Derrygonnelly in 1877:
To avoid any error or delusion on my part, I put my hands in the side pockets of my overcoat and asked it to knock the number of fingers I had open. It correctly did so. Then with a different number of fingers open each time, the experiment was repeated four times in succession, and four times I obtained absolutely the correct number of raps ['Poltergeists Old and New', SPR Proceedings 25, 1911, pp. 377-412]
The Andrews family seem to have been rather ambivalent about the case, enjoying the novelty of communicating with an unseen entity, but becoming frightened when the taps and raps turned into loud bangings, especially when they went on for hours and deprived them of sleep. By Colvin's last visit it seemed to have faded out, however. While the family treated Eric as a deceased spirit, Colvin's view is that no discarnate entity was involved, and that the case fits the pattern of repressed emotion in the living, although there was no outward sign of this, the family being apparently happy and stable.

Of course none of this would convince a sceptic: it's hard to share an investigator's conviction of the paranormality of an event without copious reassurances, diagrams, descriptions, signed statements by witnesses with impeccable rationalist credentials, and so on, and probably not even then. But my understanding is that sceptics actually never get that close to the phenomenon, in real life or even in books. If you look at the debunking literature you will quickly find that there are two main sources: James Randi's article on the Columbus, Ohio case of 1984 and a clutch of cases mentioned by another debunking magician Milbourne Christopher in his book Seers, Psychics and ESP (1970). Neither of the magicians witnessed anything (the families concerned would not let them into the house) and in any case they do not really involve this rapping phenomenon.

I'd be interested to know if debunkers like Joe Nickell who rely on these two sources to such an extent have any sensible ideas about this, beyond insisting that the teenagers are playing tricks, and that everyone else is too dim-witted to notice. Considering how insistent they are that the Fox sisters case was a hoax, and the mileage they get from it, it's a contribution they should be encouraged to make.