the chain tensioner is pumped by oil pressure but also holds by a ratchet pawl once the pressure is removed - That ratchet pawl can wear out and cause very rough startup noise and even catastrophic failure if the chain slacks and causes a slip on the cam timing (not quite common with roller chains like mercedes, usually seen with morse chains)

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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)