I actually dont own a Golf 1.9 TDI myself, but a friend is having issues with his one. My friend initially described the sound to me as an ambulance siren type sound. This puzzled me so he brought his car over for me to have a listen.

Upon hitting boost at just over 2500 rpm, there is sometimes an audible hissing noises. I myself arent very clued in on turbo's or diesels for that matter of fact, but I'm guessing there is a chance of boost leak.

Anyone have any ideas on what other possibilities it might be? It's being booked in to get checked out, but any explanation would be nice.