I have got a high pitch squeal coming from the exhaust area when driving at between 4-5krpm on my 2002 UK STi. With this I dont seem to be getting full boost until about 5-6k on the rev counter It sounds like someone blowing over a pen top
The guys at Bulldog twyford have looked at this and say the squeal could be the baffles in the back box (prodrive) or the wastgate. They also say the read out of the oxygen sensor is wrong it is reading more like a squarewave than synusoidal.

Could the oxygen sensor cause it not to boost correctly?
And could the exhaust baffles make such a high pitch squeal

Other than that just fitted an STi panel filter and forge recirc valve and appart from the other snags it sounds and drives a bit better.

 

has it been happening before you fitted the recirc?? might be something to look at, if its leaking then you wont get full boost till later and it will whistle as it releases the pressure. Personally i have never heard of the baffles making any noise other than the usual rumble, but then i dont have much experience with it, but i dont see why it would. if you changed the air filter and had the pipe work off then it might be worth rechecking that they are tight.

 

All this was happening before I fitted the resirc and panel filter.

 

I meant recirculation valve.