Update: it's back. It's been stronger and more annoying than ever but I am getting good at ignoring/living with it since I can't identify it, lol. I can hear something that sounds like an idling motor and the vibration/rumbling is in waves (just like you'd see in a graph--low then high then low etc). Wtf could this possibly be?! I'm thinking it can't be our neighbors, because if it's this loud here then wherever it originates has got to be awful. I'm thinking it something to do with our house, I dunno though! Keep the guesses coming haha. It's not localized to just one place but in some rooms it is worse than others.
Guys, I think I figured it out...I think it's my neighbors pool or hot tub filtration unit. I have no idea how to tell them to turn it off or get rid of it, I think they just won't give a crap. Wonder if I'd have any legal options if he doesn't get it fixed or whatever.