Morning mate had the car maybe five month didn't hear the sound when I purchased it but then again i didn't reverse it. Manual gearbox yes and the sound when I turn the car off is like something is open and you can hear like a rotation noise that slows to a stop pretty fast almost like a dry throat lol sorry its really strange to explain. So the car is like this always but in reverse its almost like its had a decat you know that noise they give off loud exhaust sound but it's a standard car. I did think possible regen but it doesn't fit the bill the fans come on but thats a total different noise... its like something is spinning down to a stop its strange. Ermmm i would say the ventilation doesn't effect the noise either way mate. I was starting to look into possible egr or turbo but there's nothing to go on... the car runs really well I just hear it in reverse and on shut down reverse is far worse.