sixto, I think that's the part I was talking about. Yes, the sound was more like knocking. He took a wrench and set it right on the top of the shock and the knocking sound went away. So I guess it's really the belt shock. I'm still learning about this engine...

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1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.