The rattling sound may be the worn bushing holding the shock to the shock mount...that is a fairly simple and less expensive fix than replacing the entire unit.
I've not heard of any belt tensioners failing in such as catostrophic manner as mentioned in the previous post. More than likely, the shock will wear out and the entire assembly will no longer be adjustable when a new belt replacement is in order, or a component sharing the belt needs to be replaced.
It is a moderate DIY job to perform, but time consuming, mainly due to the lack of open access to all of the components and fasteners involved. Some areas of the tensioner can't be readily seen, and have to be accessed by feel.
If a water pump replacement was in order, the task would be a bit easier...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car