Two days ago I replaced my K03 turbo with a brand new K03 turbo from ECS Tuning because recently I've been hearing a high pitched hiss sound whenever the turbo kicks in. I already know that the turbo makes a slight whistling noise when it is in operation. I did a vacuum leak last month with the redline smoker to see maybe if there was a problem with the hoses, which there was and which I then replaced. I just don't know what it is anymore. I have no check engine light and the gas pedal feels abnormally stiff. The hissing noise usually starts to show its ugly face around 3000 to 4000 rpm. I have changed all oil and coolant lines to and from the turbo and even replaced the n75 sensor to no avail. I also recently replaced the diverter valve to a R1 DV from APR. Please someone help.

Here is a video of the hiss:

 

anyone?

 

I have the same issue,but I think I found the source to my leak an its coming by the intake on mine.

 

Try checkin the boot that goes to the intake manifold its just a guess but it sounds like the boost its self is leaking