Mine had the same noise only at idle...it was the tensioner shock bushing. It only does it at idle just because that is the same frequency as the shock happens to resonate at.

You might want to consider new motor mounts when you replace the tensioner shock as I have heard that bad mounts can cause premature failure of the shock/bushings due to the extra vibrations being transmitted through them from the way the engine shakes more at idle.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz