A Audi A6L car with a distance of about 16 thousand KM. The engine of the car has a different noise. Fault diagnosis: (1) open the engine hood, idle speed found that the rear side of the engine has "gurgling" sound. VAS505 X was used to detect undetected failures. (2) query TPI has no relevant documentation on the fault. (3) audible sound can be heard at idle speed, near the left side of the timing drive chain at the rear of the engine. The car rises to listen to the sound of the connection between the rear and the gearbox. The analysis of the positive time of the 2. 4L BDW engine is driven by four chains in the rear of the engine, the structure is compact, the assembly is more complex, and the sound may be on a chain drive. (4) the inspection of the engine, before the maintenance of the same sound from the same sound from the left or the right part of a camshaft tensioner made from the sound, focus on the two tighteners, found that the right tensioner will not return to the position of a card phenomenon, so a replacement. (5) good test car discovery failure still, again carefully analysis and sound diagnosis or to determine the sound in the chain part, and re dismantle the engine, thoroughly open the end of the chain of the end cover, carefully check each part, the chain as shown in Figure 1, found that the middle camshaft drive chain tighter is relatively loose, not a little tighter. The role of this regulator is suspected, there is a problem with the tightening device. After the new accessories are sent, it is obvious that the original vehicle is missing one component. Troubleshooting: change the chain tightener.