Wiktionary
whistle note
n. (context music voice English) A note extremely high in the register, but not falsetto.
Usage examples of "whistle note".
A harmless enough remark, but enough to shatter Hornblower's frail happiness, to darken the brightly-lit room, to set the babble of conversation in the room roaring in Hornblower's ears like a torrent, through the din of which Barbara's words pierced shrill like a whistle note.