I don't have an answer for you, but I feel for you! When I lived near the ONT airport, maybe 3 miles south, I used to hear the same thing at night. I went outside too, always louder in the house. Never figured out what it was, and after a few months it stopped. Sometime last month, I heard a pretty strange radio show, Coast to Coast, coming home one night, and they had this guy on saying the gov't is tunneling underground, making a faint but annoying low frequency noise, from one end of the nation to the other, for when life on the surface is destroyed in any number of manners. . .
But, you know, living near an airport. . . kinda an obvious place to build a tunnel. . .

My neighbor now has a generator (I think) and an air compressor that makes a low frequency rumble every now and again (tho not as low as the Ont. one). Good luck with your quest in finding an answer. You have my sympathy!