I have bought a Jan 2003 A6 2.5 TDI CVT, 105k miles. The engine sounds like it has a single cylinder rattle at idle speed when it is cold that almost goes away when warm. But if the revs are increased to about 2500 rpm when warm the rattle returns quite worryingly loud. Listening through a screwdriver on the cylinder head all cylinders sound similar. Without the screwdriver from above it seems louder between the V than either side of the engine?. But the rattle is more obvious when listening under the car at the front. So I'm guessing it might be bottom end?, or could an old timing belt tensioner make this much noise?. I have a doubt that the timing belt (and therefore tensioners) were changed when due at the 80K service, so could that have caused a problem that makes the rattle?. What is the fault most likely to be for this engine model? Also, it has slightly milky coolant, and I was told it might be the oil cooler or a head gasket. The red coolant is slightly milky, and I'm told it has had a flush. Don't know if this has any influence on diagnosing the rattle?, and it might be a red herring'. Finally, there is quite a bit more wet soot from the right tailpipe than the left, might this point to a potential answer?. Both pipes exit the same box on left of the car, but I don't know if the cylinder banks have separate exits?. Any good steers appreciated, thanks!.