When my waste gate (T5) starts opening, it makes a screeching noise. That's what the Volvo tech's diagnosis was when we test drove the car. He was not sure what causes it though and did not want to start guessing without the shop foreman present. I will try a new boost solenoid and check for leaks, but meanwhile, could the actual waste gate be broken? This started in very hot weather a week ago and has not gone away.