The Taos Hum

The 'Taos Hum' is a low-pitched sound heard in numerous places worldwide, especially in the USA, UK, and northern europe. It is usually heard only in quiet environments. The low hum - between 30 and 80 Hz on the frequency scale - is often described as sounding like a distant diesel engine idling in the distance. Since it has proven indetectable by microphones or VLF antennae, its source and nature remains a mystery. Not everyone can hear the low-pitched hum, and those who do say that it seems artificial in nature - and is driving them crazy.

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So that's what it is.  Anyone who's lived in the far north of the Americas (where it's quiet enough to hear the slightest creak in the floorboard) knows it well.