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Industrial sounds, probably from excavation mining or drilling, most likely gas or steam release, reflected off an inversion layer in the atmosphere.
The sonic equivalent of a mirage. Lots more industrial activity in the canadian north, and it's all very loud. Cold dense air flows over a pocket of warmer air and it creates the equivalent of a giant drum membrane a mile or two up in the air, that catches and reflects, and amplifies like a drum body or guitar body, loud noises.
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