thats a lash adjuster and this problem is a plague on the M112 and M113 engines. They are available separately as spare parts but take some time and a pulling jig to repair as most of the times the upper tube that carries the arms gets plugged with RTV worms which then causes a lifter to suffer from no oil.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)