Sorry I can't answer your question but if you do discover what the problem is please post your findings.  I have the same problem with my Scenic 1.9DCi - no real loss of power but a whistling / squealing noise when the engine is under pressure e.g. going uphill or accelerating.  I don't want to replace my turbo and then find out it is something else.
                 John R
                
 February 2010