I've just sold my A3 to my brother, but it's got a quirky noise that I want to make sure is not a major fault.

When you let off the accelerator when there's peak boost there is a high pitched 'coooo' noise, as well as the high pressure air release noise. It sounds ridiculous but the car has always done it. Audi actually put a new turbo on recently, stating it was noisy, but it's still doing the same thing. Is this normal for the 1.9 engine?