The whistling sound used in this example is called "Whistle for Attention" and can be found at soundbible.com. The following is the spectrograph created from the .wav file:
Note the rise in the inflection at the end of the whistle consistent with the actual sound at the .wav file. The line along the bottom in the narrow spectrographic range acts as a symbol that can be considered a "character" in the whistle alphabet. Various whistles that change in pitch, up and down. or the reverse, constant, falling off completely, or pulsed, all have a particular signature that would, in effect, form a lexicon of whistling sounds, some iconic and representational, others metaphorical.