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.

Stopping after about 30 min would possibly be consistent with some sort of geophysical testing.