Could it be registered with equipment that was designed to pick up low frequency sound? Is the trouble with tracking down this stuff is because it's hard to get directional readings and triangulate low frequency sound? (The reason it doesn't matter where you position a subwoofer in a surround sound setup).
[0] https://www.thetimes.co.uk/article/weather-eye-a-resident-hu... [1] https://www.flickr.com/photos/dsf/14958914130/