I've diagnosed a few cars that had metallic clicking noises when turning the steering lock-to-lock. I think they were both E60's, but the noise was actually coming from the tension strut/thrust rod bushing area. I replaced the complete tension strut and the noise was gone.
The best way to diagnose a problem like this is get a stethoscope and try to get someone to replicate the noise. Sounds like this can travel through the suspension parts, but usually you can tell where it is loudest -- which is the source of the problem.
I've also seen a few issues on E90's where there was noise from the front strut area due to improper assembly. I'd make sure the spring is seated properly in both seats and that everything is in the correct order. Could be a missing washer or even something loose since the car was just worked on.