Moon farts
New Scientist has a
brief recording of fish making choral noises. NS compares it to birds. Sounds more like a housefly at Taco Bell.
However! It also sounds like the formerly famous Pascagoula River Hum. This supposedly happened around midnight, mainly at a full moon. Fish certainly respond to moon phases.
I don't think there's an actual recording of the Pascagoula, but good old
Strange As It Seems gave us an orchestral imitation. Definitely the same notes and harmonies.