Wife's 2012 ML-350 with the OM642 did something really strange. On first startup today it started knocking, with a moderately heavy knock you could clearly hear over the normal diesel sound. I had her pop the hood and I took off the plastic engine cover. I quickly isolated the sound to a silver (aluminum) colored part on the top left of the engine (see attached pictures, the pencil is pointing to the part) I put my hand on the part and could feel it rapping internally in time with the knocking sound.

After the engine ran about 3 minutes the rapping and knocking sound became intermittent, and 5 minutes after starting the sound and internal rapping were both gone.

Can somebody ID the part in question, and tell me how difficult and/or expensive it is going to be to replace it if it keeps acting up?

Many thanks

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)