[sEe]

My intuitive guessing:

Remember the sound of the giant device starting up in Total Recall? Imagine the sounds of machinery starting up and the sound being piped out to the surface, or even as vibrations traveling through the ground and into the air. If real, the underground bases were being prepped for full occupancy as standard procedure, so there was a lot of activity in that regard worldwide over the last few months. For some of these facilities, although totally self-sufficient with high-tech air scrubbers and life-support systems, completely sealing off these bases wasn't supposed to happen until closer to the main event. Some of the noise, as David learned and indicated, was the overpressure of explosions rushing through the natural and human-made vents which are all hidden on the surface. Sound bounces easily and certain wavelengths can travel very long distances making it almost impossible to locate the source unless you're right on top of or right next to a vent. Of course, once something real like this is mentioned, you'll always have people wanting their concocted hoax or recreation-debunking to be passed off as viable, just for the ego trip.