That’s infrasound, sounds that are below 20hz, or the limit of normal human hearing. Things that produce infrasound in nature include severe weather, earthquakes, volcanoes, tigers, alligators, rhinoceros; also known as things that can kill people. We developed an evolutionary sense of dread when our brains perceive sounds we cannot hear. The vibrations from infrasound can also vibrate the eye causing visual hallucinations.
You know what also causes infrasound? A LOT of machines, especially large industrial ones. There’s a reason haunted house stories started popping up in post industrialization. That scientist was Vic Tandy and he wrote about it in a the paper Ghosts in the Machines
“Vic Tandy, experimental officer and part-time lecturer in the school of international studies and law at Coventry University, along with Dr. Tony Lawrence of the University’s psychology department, wrote in 1998 a paper called “Ghosts in the Machine” for the Journal of the Society for Psychical Research. Their research suggested that an infrasonic signal of 19 Hz might be responsible for some ghost sightings. Tandy was working late one night alone in a supposedly haunted laboratory at Warwick, when he felt very anxious and could detect a grey blob out of the corner of his eye. When Tandy turned to face the grey blob, there was nothing.The following day, Tandy was working on his fencing foil, with the handle held in a vice. Although there was nothing touching it, the blade started to vibrate wildly. Further investigation led Tandy to discover that the extractor fan in the lab was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye given as 18 Hz by NASA. This, Tandy conjectured, was why he had seen a ghostly figure—it was, he believed, an optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the centre, thus causing a standing wave which caused the vibration of the foil.“
