3.0L v6 Bluetec diesel Engine rattles upon cold start for a few seconds. I found out that noise is due to a stretched timing chain which is caused by light viscosity motor oil recommended by manufacturer Mercedes-Benz. This issue might result in engine failure and replacement could cause 20,000 USD. A timing chain replacement costs anywhere from 2000 to 4000 USD. There are several owners on Mercedes forums complaining about this issue with as low as 40k miles on their cars. There's no recall or service bulletin issued by Mercedes-Benz so far. This issue also makes the engine run rough with poor performance.