Mercedes C220d timing chain rattle help needed

[Bluesky322]

Hi all
I have a 2012 mercedes C220 diesel and just recently sometimes on cold start the timing chain rattles until the tensioner kicks back in,my question is where is the tensioner located and how hard is it to replace on its own as although I paid good money for this car only last year it would not be cost effective to take it to a garage and have the timing chain and tensioner replaced.
Many Thanks