10% of the Population Hears an Unexplainable Hum

Do you hear an inexplicable low-frequency hum in the evening? Congrats; you are one of seven hundred million truly awesome people!
Earth likes to mess with us. For some reason there is a "frequency of around 10 millihertz pulsating around the world," which most humans shouldn't be able to hear, yet 10% of the planet's population does (quote here). It sounds like a "distant idling diesel engine," gets louder indoors, and is not affected by earplugs (quote here).
Why does Earth make that sound? How is it possible that humans can hear it? Do I wish I could hear The Hum? We don't know, we don't know, and yes, yes I do.
You can find a replication of The Hum here, but it can be very uncomfortable to listen to. I listened to it at a pretty high volume and didn't feel quite right for a few moments. (Yet I still wish I was one of those seven hundred million; go figure...) I suggest you listen to it at a low volume to dilute the eeriness.

Do you remember hearing strange trumpeting from the sky in 2011? (I don't, but you can't blame me--that was ages ago!) This occurred three years after scientists discovered what causes The Hum. These sounds were almost exactly aligned with The Hum, yet whole cities from all over the world could hear them. These loud noises were described as sounding like rusty trumpets. We still don't know what caused it.


Resources:
http://www.travelcreepster.com/the-hum-travelcreepster.html#.U04EaqhX-uY