I have this exact issue too, repeated trips to the dealer, replaced CAT and turbo and its no better.
They are 99% sure its the actuator and there is nothing they can do. I have went as far as BMW head office and I get the 'Characteristic' of the car excuse which is a disgrace.
However, looking on here and Babybmw and talking to the dealer shows im not alone.
Now its the difficult problem of determining exactly what is causing the noise, collating responses to this issue and either sussing it ourselves or going back to BMW in a group and complaining.
Just for info I have had loan cars recently doing the same thing so unfortunately im not sure they have solved it at all
