could be the turbo making metal/metal contact, with the engine off, how much end float is there in the turbo shaft, is it like a d1ck up a shirt sleeve..?

other favouite is a split house that is only opening up under boost and letting a small amount of air leak, maybe inlet manifold gasget..

a good way to find leaks is to have the engine running, and geta can of carb cleaner. spray around all the joints , hoses, manifolds etc and see if the engine revs up, if it does, youve found the leak