The sub-audible motion of the wind made audible.
Windharp naturally played by the wind.
Story behind the song
When the wind is very light or non-existent, sub-audible portions of the audio band can be recorded from the windharp. Some sound is generated by wind motion and some by infrasonic tremors of the ground. To hear them, the pitch needs to be increased to the audio realm. That is what you are hearing.
