Melbourne, Australia (I added more details above). Thanks, that sounds like it exactly! So, I'm not going crazy (or, at least, not alone in it). I'd guess it's

Melbourne, Australia (I added more details above). Thanks, that sounds like it exactly! So, I'm not going crazy (or, at least, not alone in it). I'd guess it's related to industrialization in some way, as low frequency sounds can carry long distances, and are difficult to directionalize.