pilot injection makes those engines sound a bit like diesels. a wastegate rattle sounds different. tends to disappear under load and reappears when you take the foot off the throttle. a bit like this - i used an extreme example so you don't need to second-guess what type of noise you're looking for:
https://youtu.be/DsKLNCRaBv4
on N54s you can adjust the actuator linkage for the wastegate, not sure about N55s but i'd be surprised if they did away with that. it could also be the solenoid that operates it. replacing the turbo should be your last plan of attack, really.