We have a 2005 A4 1.8 automatic that has a rattle when it is idling in gear. It comes from the area of the belt tensioner and you can see the tensioner vibrating back and forth as it keeps the proper tension on the belt. We have had it repaired at a general repair shop and an Audi "expert" looked at it. They replaced the serpentine belt and put in a new generic tensioner. The noise did not go away. They put in an Audi tensioner and the noise is still there. I think they put in a slightly shorter belt, and the noise is still there. They checked all the bearings/pulleys that the belt goes over and still the same. The vibration is mostly when car is sitting, idling, and in gear. As soon as you accelerate the nois goes away. Has anyone experienced this? Thanks for any ideas..