For a long time, a low-frequency hum was reported around the world, the source of which could not be identified using external instruments. In most cases, researchers found that this phenomenon was linked to a specific type of noise generated within the human auditory system. This conclusion was reached by scientists studying the nature of the mysterious hum.

The strange hum was first recorded in the 1970s in the British city of Bristol, and later reports came from the USA, Scandinavia, and other regions. According to statistics, between 2% and 4% of the world's population experience this phenomenon.

The hum is most noticeable indoors and during nighttime silence, causing people to feel physical discomfort and a sensation of internal vibrations. Attempts to find external, man-made sources of the sound have yielded no results.

To investigate the nature of this phenomenon, a group of researchers from the Norwegian University of Science and Technology conducted a detailed examination of the auditory systems of people who experience the hum. The results of their study were published in the scientific journal PLOS One.

During the research, two main hypotheses were tested: whether participants had hypersensitive hearing capable of detecting low-frequency vibrations, and whether the inner ear could generate sound itself (otoacoustic emission). Neither hypothesis was confirmed. Only two individuals showed minor sensitivity to the relevant frequency range, while most participants had normal test results. Measurements using sensitive microphones in the ear canals also did not detect any physical sound vibrations produced by the ear.

The main conclusion of the study is that the mysterious hum is a form of subjective low-frequency tinnitus—a phantom noise that arises within the human auditory system and does not come from external sources. Due to its extremely low frequency, the brain perceives this internal signal as real vibrations from outside objects, such as a running engine, ventilation, or a heat pump. While rare cases of actual man-made noise are not ruled out, the widespread occurrence of the hum is associated with the peculiarities of human hearing.