I think in some cases it is the sound of drilling equipment underground digging tunnels. Sometimes it echos off hills and valleys and you get different pitches in different reflections so it sounds “choral.”
In the 1950’s they were stripmining in the hills above our town in east Ohio. The huge diesel machines dug into the ground at full throttle & made a galloping noise that echoed at night. Older kids would frighten younger ones by saying “Here comes the Headless Horseman!!’
Glenn Beck discussed this at length today. One caller said it was the Lord sounding His shofar.