I'm not certain what's wrong with my 159 2.4 Diesel. This video was taken a year ago but the noise went away - I thought it was the reed effect from a leaking exhaust manifold gasket.
Cold start. - YouTube
It's not power steering, alternator or air con - nothing driven by the auxiliary belt - I dropped it off and the noise was still there. Cam belt, water pump and tensioners are all less than a year old.
It only does it when cold, or when the exhaust has cooled from a long downhill run off the throttle.
I did have the two studs sheared off no5 exhaust port, but they've been drilled, tapped and new studs fitted. Fitting one seemed to stop the noise last time.
The noise does stop if I disconnect the vacuum line to the turbo - now this will do two things, lower the boost pressure and move the vanes away from the impeller (I believe).
I have had my hands on the compressor wheel and there is no axial movement along the length of the shaft, there is only the tiniest radial movement when pulled up or down. The compressor wheel does not feel as though it fouls the housing when turning, even if pulled in one direction or another.
I am still of a mind that it may be one of the metal gaskets resonating due to an exhaust leak, due to the very shrill noise and changing tones.
Anyway, have a listen and see what you think.