If Scooby Doo has taught us anything it's that if you dig deep enough all ghost sightings have a rational explanation, and you shouldn't trust hotel managers.
In 1998, a group of scientists working in a laboratory at a medical equipment company began noticing strange events. One of the scientists, Vic Tandy (shortened to V.T. in a report about the case) first witnessed a cleaner leaving in distress, claiming that she had seen something inside the lab. Being a rational engineer, Tandy put it down to an animal – there were wild cats around the company's premises – or an effect from some manufacturing equipment. But the odd events at the lab, which was rumored to be haunted, kept coming.
Workers had feelings of depression and unexpected shivers. There were other little events, like a colleague who turned to talk to Tandy, believing that he was right there, only to discover that he was on the other side of the room. Then late one night, Tandy began feeling increasingly uncomfortable.
"It was as though something was in the room," a report into the case, published in the Journal of the Society for Psychical Research, reads. Tandy checked bottles of carbon dioxide and oxygen on the site, to make sure that there was no leak, and returned to his desk. "As he was writing he became aware that he was being watched, and a figure slowly emerged to his left."
The gray figure remained in the periphery of his vision, but to Tandy it appeared to be moving like a person. He plucked up his courage and turned to face it, at which point it disappeared.
All very terrifying, but Tandy didn't have to wait long for a rational explanation for all the strange events. The very next day, Tandy used the lab for one of his own projects, which involved keeping a foil blade in a vice. When he searched for oil for the job and returned, he discovered that the blade was vibrating wildly up and down, once again scaring the crap out of him.
"However, vibrating pieces of metal were more familiar to him than apparitions so he decided to experiment. If the foil blade was being vibrated it was receiving energy which must have been varying in intensity at a rate equal to the resonant frequency of the blade. Energy of the type just described is usually referred to as sound."
Tandy began experimenting, moving the knife and vice down the lab to see where the vibrations were strongest, finding that they peaked towards the center of the room. He determined that something in the lab was creating infrasound, a low-frequency sound wave below human hearing.
Asking around, he traced it back to the source: a new extraction fan. Switching it off stopped the low frequency noise. Searching around the literature, he found similar reports of unease and oppression where infrasound from fans had been determined to be the culprit, as well as a NASA report noting hyperventilation as a symptom of whole body vibration.
Subsequent studies, including one that deliberately set about creating a "haunted room", have shown evidence that infrasound can produce effects similar to hauntings in humans. Of course, not all hauntings can be explained in this way, with other explanations ranging from hoaxes to carbon monoxide poisoning, and the genuinely unknown.