[urq] Wastegate "Chirp" is *not* a myth? So say the boys from UK and CAN

[lawson.d at comcast.net]

To answer the original question, it's the sound of the boost pressure spike wavefront propagation and reflection off the closed throttle plate through the intake plumbing and back to the turbo (causing compressor stall) and back to the throttle, repeating the cycle as energy is lost and the wave slows down. That's why the frequency of the sound decreases with each reflection. 
- 
Dave