This is a flageolett sound played with plectrum on a steel-stringed acoustic guitar. I used this sound for coursera course "audio processing for music applications".
This is the sound of the spring in my step now go forth and use it to lighten the load. . . . . . . Space cadets !!!!. (-:-). . . . . . . . . . . . . /\. . . . . . . . . . . \0/.