One of the stranger HF oddities is the "Whale Sound Station." It always appears near US military frequencies, and it makes these haunting, descending moans that have been likened to humpback whale sounds. It's a bit like the "Backward Music Station" heard in Europe, but far more complex, and showing no ring modulation.
Here we see that the "whale sound" is concentrated around several evenly spaced frequencies between 300 and 900 Hz. Note the interval between 9 and 10.5 seconds, where it actually gets six tones going at once. I doubt, personally, that a drifting LINCOMPREX pilot could do this. Ringy landline circuit noise has also been suggested for a cause, as these links are very heavily conditioned, and the "whale moans" die down for a minute or two after someone transmits on the frequency, briefly lowering the compressor gain.
For whatever it's worth, a song called "Electric Cafe" by the German synth band Kraftwerk has a similar sound in various places. Coincidence, or ???????
All plots made with GRAM.EXE.