I've a 53-plate BMW E46 3 Series automatic diesel coupe (the 2-litre 320) which has taken upon itself to develop an annoying rattle. It doesn't lose power or emit smoke, and it improves when the engine has warned up. The oil level is fine. I've spoken to my usual garage and it's booked in for a service shortly, and I've gone over the above with them and they didn't suspect or suggest anything particularly, other than agreeing it probably isn't a blown turbo as that would be a lot more obvious. Is there anything obvious I should be getting them to look for?