Mystery humming noises have plagued the inhabitants of various locations over the years, from Taos, New Mexico, to Largs in Scotland to Bristol. Explanations have varied, often involving the electrical grid, power plants or military experiments.
But in the case of Sausalito, California, where a community living on houseboats was driven mad by a persisent hum for several summers, as described in this great BBC radio series The Sound of Life: The Underwater Chorus, the mystery was finally solved when biologists decided it was caused by a fish. The drone caused by a gathering of male midshipman fish all competing for mates is truly uncanny:
Not only do they hum, but they’re also bioluminescent. You can hear more of their sounds here