Thanks for the quick reply. The car is parked, some people can convince thereselves that a noise like that ain't going to hurt anything but I can't. I turned the motor over and looked at the chain and all the links look to be peened the same so can I assume that this is the original chain? 351,000 would be alot of miles so I guess it should be getting loose by now. I am going to replace the timing chain, the tensioner rail, the pin at the bottom of the rail, and maybe the tensioner. And hope the noise goes away.

Is there any way to check the tensioner to see if it is working? or is it just one of those things that you replace if in doubt.

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1993 300D 2.5 Turbo (blue/ blue tex)
1991 350SDL (White/ Gray leather)
1983 300TD Turbo 5-speed manual (Green/ Beige)
1985 300SD (Black/ Black Leather)
1985 300TD (White/ Green Tex)
1980 300SD (Astral Silver/ Black Leather)
1990 560SEL (White/ Gray Leather)
1993 300SD (Black/ Black Leather)
1967 200D (Green/ Beige Tex)
1969 300SEL 6.3 (Moss Green Metallic/ Green Leather)
1975 300D (Astral Silver/ Green Tex)
2001 Ford Excursion Diesel