I have replaced my tensioner shock twice on my 350 SDL - wear rate is about 2 years max.
While some of the threads point to a bushing getting soft or othewise deteriorated, the actual shock absorber mechanism in mine wears out ...
The rattle is pretty horrific ... put a new one on for about $25 and presto .... (relative) silence.
First one took an hour ... second one about 20 minutes.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg