Guess the name of the low-pitched sound heard in numerous places worldwide, especially in the USA, UK, and northern europe.This sounds similar to the distant sound of diesel engine but is undetectable by microphones or VLF antennae.

The Hum is an unexplained low-frequency phenomenon reported in various locations worldwide. The Taos Hum (New Mexico, USA) is the most famous case, first widely reported in the early 1990s. Similar hums have been documented in the UK (Bristol Hum) and other places. The phenomenon is known as 'The Hum' or by location names (Taos Hum, Bristol Hum), not 'Tao's Hum'.