I had experienced that same noise when the car would sit for a few days without being started. The wife complained about it so we took it in while still under warranty. The dealer repalced the Kompressor Clutch and waited for a few days to checke it then replaced the Kompressor. It still makes the noise when started after it sits for a few days, the longer it sits the louder the noise. It is no longer in warranty and I have learned to live with it. We still like the car, just wonder if it is going to self destruct some day.

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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 Sold
2012 GL450 67K
2016 E550 Cabriolet 48K