Here's my latest, based off of a story in Mermaids: The Body Found (you can watch it on Animal Planet, it was pretty good). Yes, the main character is a mermaid. The sound is military weaponry testing. It's a supposed weapon that can kill with deafening sound waves that cause vibrations in the body so strong it can cause blunt force trauma.
Of course, this is all conspiracy theory stuff,so I'm not saying any of it is true n the slightest. It just made a good story is all~