It's the
MLB spying on them - they should coat their entire house with aluminum foil and metal coat hangers.
On a more serious note, it could be anything from water hammering in pipes in their walls, to anything with a motor, like their refrigerator, air conditioners, water/sump pumps, pool pumps, or a thumper built by a neighbor obsessed with Dune. Or depending on how close they are to a railroad, highway, power plant, water/sewer pump station, or any industry that uses diesel engines, if the wind is blowing just right, sound can carry on for miles. Or they have a wicked ground loop hum in any speakers they have in their house (which at 60 Hz, would probably sound like a low engine rumble).
Or they are just imagining it.