I have been listening to the most intimidating rumble(seemingly coming from the sky) for almost 3 - 4 hours. Now, I am not one to panic or who believes in the weird theories that many believe - ghosts, aliens, etc. Had I not heard this for myself - and so do my kids and neighbor - I don't know if I'd believe it. But it sounds like a jet engine hovering overhead or a long sustained thunder rumble or even an earthquake in the distance. There are no weather issues. Yes, we are not far from the Philly airport, but if we hear rumbling from there it usually lasts only a few seconds. I can still hear the plains when they fly over. They do not sound like that rumbling noise. This has been going on for too long. You can actually feel the rumbling. There has to be an explanation. Anyone?