Are you in a area that has gas/oil fields? They use "Thumper trucks" to sound things out to figure out where to drill. Yeah, they even do it in residential areas. I have read that some people have reported that it has caused cracks in their houses foundations. If that's what's going on in the area I suggest looking around for possible damage. If you are in a major metro... You finally hear what it sounds like? I cannot stand being in large metro/city areas for long. There is a low "hum" that never goes away. I guess people get used to it or they were born around it so they don't notice it. It cannot be healthy to be around.