I no longer have my C230 but the fix for the grinding noise was to replace the
Cam Chain Tensioner. Once that was replaced we no longer had the noise even after sitting for a week or more. Since the original started to make noise after about 40K miles I bought a spare to have when it started making the noise again. Unfortunately The car was totaled before I needed the spare part.

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Dave
88 300E 250K Sold
2000 C230K Totaled @104K
2003 ML320 156K Sold
2009 E350 4Matic Sport 46K Sold
2011 ML350 60K Sold
2014 GLK350 46k
2012 GL450 55K