Whistle flute

Wind instruments are tubes. They produce tones by taking a vibration and then modulate the tone by adjusting tube length / sites of egress from the tube via keys or covered holes or valves or a slide. The initial vibration could be from a reed (woodwinds) or buzzing lips in a brass cup (brass) or blowing across a hole (flute).

If you whistle then hold up a tube to whistle through, the whistle will generally stop. But sometimes with certain tubes / cups etc the whistling changes pitch.

The whistle flute takes at its initial vibration a whistle made by the player. The whistle flute then externally modifies the pitch of the whistle. One could adjust tones both by adjusting the whistle tone, modifying it with the flute, or both.