Sound produced by emitting breath through small aperture in pursed lips, pitch being controlled by shaping of the mouth as resonating chamber.
Some people can whistle through their teeth.
Some professional whistlers have appeared on concert platform. Bing Crosby was a mellifluous whistler. Harty, at rehearsal, did not sing or hum phrases to the orch. Term for various primitive wind instr. The Grove Concise Dictionary of Music 3.