Actually, I've got to go somewhere shortly so I'll just spit the idea out & get to the rest of the thread later.
What do we know of that readily resonates & also amplifies sound? Tubes right? Bang on 1 & you dont just hear the clang or thud, you get some kind of
boing or twang from the open end. Think also of wind instruments, foghorns & old fashioned gramophones, where the flared shape amplifies the resonance
in the thinner tube.
Ok, what do all modern cities & most neighbourhoods have in common?
Think about it: a whole series of sealed tube that goes everywhere, gets wider, has resonant chambers that lead into bigger tubes... & has a complete
network of holes in it; in principal its like a giant complex saxophone/flute with multiple mouthpieces.
There are plenty of ways that energy can be transferred to make resonance: air & water moving through it, pumps that are part of it, traffic noise &
physical vibration, wind blowing over open drains like panpipes, ELF sound from the ground etc. Most likely all of those, in varying amounts, all the
time. However, many of the sections are of the same diameter & length thus will resonate at the same frequency. At any given time of day, the
resonances of different sections could be in phase, thus amplifying them, or out of phase, cancelling them out, but any constant sound sources will
interact with each other predictably, whereas constantly changing sources will modulate the interactions, like playing the instrument.
When you think of the size of some of the tube, its not hard to realise that the output sound would be at very low bass frequency. Except that at such
low frequency, its quite difficult to hear pitch & timbre accurately, so we would most likely mainly only notice when it got louder or quieter.
If this hypothesis holds water
then its possible that the output sound is so low that it causes walls to vibrate, or perhaps it just enters
houses through drains & reverberates, thus seeming louder than outside.