The project was removed from Electronic-Lab because it does not work.
I think it was poorly translated from Greek to Engrish so it says, "fizzle" instead of whistle.
It uses a piezo transducer not a piezo buzzer (another translation error). A piezo buzzer (with a built-in oscillator) would probably keep buzzing by the DC from the transistor driver.
A piezo transducer is both a microphone and a speaker.
The problem is that the piezo transducer is very sensitive at its 4kHz resonant frequency and nobody can whistle higher than about 2kHz.
The project beeps when it hears a 4kHz smoke detector. Try it.
My daughter bought a cheap keys-finder at The Dollar Store. She wanted me to fix it because it beeped whenever the TV was on and whenever she talked. I couldn't make it beep and I couldn't fix it.