they have a very systematic approach to the rattle at BMW - just had mine fixed - before they will replace the waste gates or the entire turbo they have to replace the actuator arm. The actuator arm is not covered under the extended factory warranty (that applies to the waste gates or entire turbo in some cases). If the actuator arm replacement does not fix the rattle they will either replace the waste gates or the entire turbo (which is covered under the extended special case factory warranty).
Fortunately, I have an extended warranty which covered the actuator fix which was a 30 hr job apparently making it quite expensive job. I do not get why they don't just replace the waste gates while they have the turbos off the car...bc if the actuator arm doesn't fix the problem they have to go ahead and take the car apart again...another 30 man hr. job for some relatively inexpensive parts.