i have read some threads on this but a lot come up in my searches that are different motors

my car is a 99 e300 turbo

i have a very slight rattle in the belt tensioner shock and we were going to replace it
it looks like it has been replaced at least once
car has 170,000 on it now

should i order all the parts or just the shock?
and if all the parts, please tell me which ones

thank you

__________________
Courtney
2006 E320 CDI
2008 ML320 CDI