Does this happen only when cold and for how long does the sound occur? If its just for a few seconds (2-5seconds) on cold start up its the hydraulic lifters, nothing to worry about, 100% normal. They tend to lose their oil pressure when standing for a while, especially overnight, this causes them to make a rattling sound when the engine is started again the next morning until they have reached their normal pressure again.
I would say if they rattle for 10seconds or more then they might need a change and the head needs to come off..This can also be caused by an faulty oil pump supplying insufficient oil pressure to the top of the engine internals starving the lifters and not ''pumping'' them up correctly. Not sure if that specific engine has hydraulic lifters but I had the same symptoms in my 328i..