I live in Eatontown NJ, Central East area. Theory one, the earth is moving and the sound is bouncing back down. But I hear them too here in NJ. Theory two......I have no Theory two. Oddly enough even though i live in a highly populated area, late nights afford scilence and I can hear the constant hummmm, during the day it is muffled by planes and cars, but when you catch a break it is still there!