Spectrogram of the train
The Train is the name given to an unidentified sound recorded on March 5, 1997 on the Equatorial Pacific Ocean autonomous hydrophone array. The sound rises to a quasi-steady frequency. According to the NOAA, the origin of the sound is most likely generated by a very large iceberg grounded in the Ross Sea, near Cape Adare. 
The Train, from the NOAA
website at 16x the original speed.
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Some of the other significant unexplained sounds named by NOAA along with train are Bloop, Julia, Slow Down, Whistle and Upsweep.