Whistling sounds are produced by placing the palm of the hand horizontally over the strings and then sliding it quickly upwards or downwards. The hand can leave the strings after the whistle or stay on the strings
Whistle downwards, hand leaves the strings.
Whistle downwards, hand stops on strings.
The sound is a short "whistle"-like sound. It is not very loud but carries well. For a louder whistle it is possible to produce the whistle with crumpled sheet paper held in the hand.
Whistling sounds are notated with two horizontal lines between two normal noteheads. An arrow shows the direction of the whistle. The lower notehead shows the approximate placement of the palm of the hand in the harp's register. This notation, taken from Sequenza II for harp solo by Luciano Berio, has become the standard notation for whistling sounds.