Not familiar with your exact setup but if it is similar to my 603 there is a small shock absorber opposite the tensioner spring. This shock wears and the bushings on the ends wear out and rattle at idle. I know on mine it needs replacement about every 50K miles. Simple to replace and the shock is about $40. Hope this helps.

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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K