Around the world this seems to always occur near large bodies of salt water where rivers of fresh water flow into them.
Think of the surface of the ocean as a large bass drum. When salt water and fresh water mix there is a decrease in volume, in other words, when 1 gallon of fresh water and 1 gallon of salt water mix--the resulting volume is less than 2 gallons. Also there is a large energy interchange when two very large volumes mix. The combination of the two could result in rapid volume changes large enough to reverberate and cause sound.
Since this has been going on longer than there have been supersonic aircraft, I would rule that out. There is always the consideration of USO, Unidentified Submerged Objects.
Also consider "The wheel".
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