For two years now, the Ehrfurths have been enduring an annoying, persistent noise in their home — a low, motor-like rumble accompanied by a vibration. They can't figure out what's causing it, and it's been a challenge getting others to believe them because the problem starts and stops.
There is no mystery here. I have heard that sound myself and know what it is. it is the sound of deep earth boring machines making tunnels for the military.