Its source was never known and its not known even at this time. More, scientists say the sound is inaudible to humans, but thousands people reported to have heard it. “The classic description is that it sounds like there is a truck idling outside your home.
For some people, it is a deep and distant droning bass tone. Some people perceive the sound as a rumbling noise. The sound is louder indoors than outdoors, and louder late at night than during the afternoon. It can suddenly appear or disappear for days or months,” MacPherson – who own a website dedicated to “the hum” said. Researchers say it may be the result of tiny vibrations in the Earth called ‘free oscillations’ that can only be picked up by sensitive equipment. However, there are many hypothesis and even the presence of aliens was considered. The sound was measured using underwater seismometers positioned across the globe. An important fact: the hum only occurs in the absence of seismic activity.