wondered. with air induction, there is a kind of hiss when you back off. is this caused by the air system blowing back out, as it sounds, or caused by something else. just wondered where the noise came from?

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Sounds like the turbo winding-up or down according to throttle openings.

My old 2005 Fabia VRS lets out a high pitched whistle under heavy acceleration.

Would have thought everything is normal-no real worries.

You're flowing considerable volumes of air through fairly narrow ducts, which means you get a fair amount of air speed to get the volume flow rate you need, which means you get a lump of noise.

The hiss is probably a flow reversal transient peak, or else an artifact from the recirculating wastegate.