Apparently no one fully understood the phenomenon until 2013 https://www.cam.ac.uk/research/news/how-the-kettle-got-its-whistle How I interpret what the article said was the steam goes through a narrow opening to form a jet, the peripheral part of which hits the whistle wall, which creates a pressure pulse on the wall and cause the jet that escaped to form vortexes of a specific frequency. How does the pressure pulse on the wall affect the vortexes that the steam forms outside the whistle? This is also different from a helmholtz resonator (as mentioned in the article) as the pitch of this whistle can vary continuously depending on air flow. But why is it different from a normal cavity resonator? Which part of its mechanism differs so that its pitch can vary?