Turbo 2 help lossing boost
 

I am having a problem with my t2. It is a little hard to explain so i am going to try my best. If i hold the throttle at 4000 rpm under load in any gear it sounds as if all the boost is coming out somewhere in the engine bay. Now if i flat punch it you will here the same noise in till about 5000 rpm. Also the car has lost a lot of power all across the rpm band. Any help or ideas would be great. thank you in advanced