TURBO!!! Its normal lol, but yes this does need to be louder... first turbo car I assume?

 

I have a whistle, but it's a deeper almost jug like whistle, I dot it when I removed my intake box and did my homebrew intake...basically, it only whistles when the wind hits it just right, I am pretty sure that you wouldn't hear this noise unless you do not have your intake box in and your snorkel is still in place...

here's what I am talking about;

 

quattroa4 if that is your engine congrads on having the cleanest engine ive seen so far in one of these cars. very nice.

 

have had some similar problems. I hear this whistling or loud NAAAAAAAAAA sound at higher RPMs, usually 2.5K and above. At first I thought this was the turbo too since the problem was in synergy with when the turbo when turbo is on for accelerating and downshift accelerating. Now I have a different opinion. My check engine light came on the other day and read back code P1176, I was told this means either Rear O2 Sensor failure or Catalytic Converter failure. Given the fact my A4 is 10 years old and has 110K miles on it I wouldn't be surprised. But my thoughts are that its possible the catalytic converter is the problem becuase sometimes the turbo goes on without the noise at all, but when the noise does happen, the turbo kicks on and I hear nothing for the first 5 seconds, then I start hearing the noise. Makes me think its something on the back end and exhaust system since there is a delay on the noise. I have no idea if it could be this vacuum seal valve clogged stuff, its possible, but I dont know. Im just more sure its not the turbo itself.