What could be the source of "the hum"?

No.11709305 ViewReplyOriginalReport
It's a low frequency sound heard all over the world coming from all directions. It apparently causes people to get nosebleeds, go mad and I has been linked to one suicide and one terrorist attack. What could it be?

https://www.youtube.com/watch?v=PZSFI3vRabo
https://en.wikipedia.org/wiki/The_Hum
https://thehum.info/