Belt Tensioner Question
 

My '97 E300D with 212k miles makes a "tap tap rattle" noise when idleing that quickly goes away when rpms are increased. In looking at the tensioner lever at idle it shakes back and forth slightly ...... but becomes steady at speed as the "tap tap rattle" noise disappears. The shock/damper doesn't seem "tight" to me when I grab it and twist so I suspect that it needs to be replaced. However, how can I know if the shock/damper will fix the problem or if the tensioner lever is also somehow worn and in need of replacement? In order to replace the shock/damper I have to remove the fan & pulley and I would hate to go to that effort only to find out that I really should have also replaced the tensioner lever when I was in there. Should both units be replaced together ..... or should just replacing the shock/damper do it?