I hear the hum intermittently, but more often the lower 25hz rumble. I always assumed it has to do with mutual interference and perhaps local sound convergence. Buildings do bounce and funnel soundwaves to a noticeable degree - I live near an airfield and often you will pick up the sound of an aircraft from a completely different direction than he's coming from. And at some places in town, lawnmowers on the other side of town can be heard as if they were next door.
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