I'm new to this so bear with me if I've got things wrong!
I drive a 2006 X3 D M-Sport with 102k miles on the clock. Unfortunately, although running really well, there is an awful rattle which is most noticeable at low revs but is always there. It's a repetitive rotational metallic noise but not like tappets. I have had a BMW-trained mechanic working on it for weeks now. He has checked all the obvious - chain, guides, vacuum pump etc. but he has just about given up! Has anyone any idea what this might be, please? Any help or ideas or suggestions would be very much appreciated.