You could go out on a weird limb and say it's some kind of secret military tunnels being dug under the United States and the booms are the dynamite blasting for the tunnels.
You know, in New York City, there's constant construction of new subway tunnels several stories below the buildings and streets! People are given notifications inside some buildings to not be alarmed by the loud boom sounds they might hear (because of obvious reasons, people might get nervous if they hear explosions there).
If not some top-secret military project, perhaps it's some industrial noise source that travels through the ground because it's such a low frequency.