It Hasn't had any engine work done in the last 4 years, wouldn't bad timing components have other symptoms? My last car a 93 300E had the head gasket blow and the shop I took it to FUBARed it and installed the timing sprocket wrong but it would only make a noise after 1500RPM - 2000RPM I called a dedicated MB guy and told me to disconnected the camshaft positioning sensor?? round thing on top front of engine??? so it wouldn't advance and drove it to him who fixed that for me and billed the other shop for it.
This sound is completely different from that. Not sure if that helps or not.
Truthfully, all of us are at a huge disadvantage here and we understand you are trying your best. At this point, I'm out of suggestions other than taking the car to the trusted shop that helped you before. If you go that route, just let us know what the diagnosis is.
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