Im assuming your tensioner is the same as on the M104 engine, a ratcheting plunger pushed by a spring so that it clicks against a notch as wear occurs. Forward but not back. Now on taking my heads off and doing the RR work I found the tensioner will easily stick on the notches and when you install it must be backed off and then you take a small ratchet drive or bolt and click click..push it forward to contact the chain. Not hard, but firmly. Mine turned out great but you hear merz' all the time with clanky buzzing sewing machine sounding timing chains like my previous E320, they smooth out when the engine warms up. I suspect timing chain wear with a stuck tensioner. My chain was VERY worn but no noises BC the tensioner works well. Even some flat spots on the chain shoulders. 200K miles and likely original chain. Then you've also got the issue of those balancer shafts and other harmonics going on in those V engines..

Hmm driver's side guard..and the support bolts for that and your stethoscope?? I used to use an old phonograph pickup and a transistor radio which made an electronic listening device to keep track of my old VW's tappets. Forgot all about that! Worked great!

Good luck.