I investigated a bit further and came upon this topic:

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, and is often described as sounding like a distant diesel engine. Since it has proven indetectable by microphones or VLF antennae, its source and nature is still a mystery.

The Hum is descibed most commonly as a deisel truck idling in the distance. Sometimes the sound is constant, sometime it constantly changes in loudness at between one and four times per second. Sometimes it has a slow "beat note" sound, other times a "morse code" effect. Some people perceive it as being louder
indoors than outside.

I include a link about this subject:

http://amasci.com/hum/hum.html