IMHO: ...FYI: I've DEFINITELY heard this DEEP, "RUMBLING" "sound", ....I heard it while camping, and sleeping, on the ground, in the South West section of the Boundary Waters Canoe Area Wilderness, in 2000. I thought that it was some sort of noise traveling thru the ground (solid rock), from deep below the surface, (possibly underground tunneling (an Underground Tunnel Boring Machine), or mining,... ... or, perhaps, it may have been the low frequencies from distant trains,... which would've been many miles toward the West,.. but, as I listened, carefully, I was convinced that the CONSTANT, LOW, "RUMBLING" sound was coming from DEEP, BELOW GROUND, not from any direction, above ground!... I've NEVER forgotten this event!!! ....WEIRD!!!!!!! The Controversial Sound Only 2% Of People Hear https://www.youtube.com/watch?v=zy_ctHNLan8