I recently acquired a nice 2006 R53 rally edition with 150 000km, with a few minor issues. One issue is at start, for about 5 seconds or when I rev high, we can really hear the timing chain rattle. It will rattle and slap badly for a few seconds, and then suddenly stop, it'll work great but at high rpm's, we can hear the rattle begin slightly again. needless to say the car is parked until this is fixed. Originally friends and i figured it was a bad timing chain or guide, however, reading on, I see lots of people have had issues with the oil pressured tensioner.

 

Replace the tensioner first and see if that works

 

Replace both. Chain tensioner is cheap and very easy to replace.