That sounds like the timing chain hitting a guide or something. Maybe your tensioner is failing. I'd do the entire chain set if it is determined to be the culprit. I'm not sure about the m103 but the m110 looks very challenging to complete. You can use a piece of hose to help determine where the noise is coming from by sticking one end to your ear and the other end to the suspected area. The chain would be making noise in the front of the engine only.

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Chris
84 280sl
82 300d euro