Hi.have had this for a while now. Cars a BA xr6 turbo sedan, factory, besides a cat back exhaust and a BF tyfoon cat. When I giive it a bit and back off the car used to have a nice flutter, but now it seems to be a belt screech type sound.... sometime it does have a flutter but other times there no flutter just teh screech which last for maybe half a second. I have checked for boost leaks, found none, im thinking the factory BOV?? my mate who has owned allot of turbos cars (sky lines, supras wrx etc) said it sounds like the front fan on the turbo loose but should that be there while boosting? as the turbo spools up perfect and no screeching, I do have a very loud groaning sound at idle but im almost certain its from power steering as its always been there. mayybe the belt is slipping? it has been on car for 20 000k's, or idle tensioner assembly? cheers