What causes turbo hiss?

Turbo hiss, often characterized by a high-pitched sound, is primarily caused by the release of pressurized air from the turbocharger. When the throttle is closed, the excess air that the turbo has produced has to escape, typically through the blow-off valve or diverter valve. This rapid release of air creates a distinctive hissing noise. Additionally, the design of the intake and exhaust systems can amplify this sound.