A mileage B7L 2.0T car with a mileage of about 42 thousand km and 6 double clutch transmission is assembled. The owner reflected that the car was running near the left front wheel on the flat road, and there was sometimes no sound in the tower. Fault diagnosis: 1. normal road interview car, start normal, no abnormal noise. The abnormal sound of the 2km appeared, and the sound in the driving room came from the left front wheel, the left front transmission shaft, the transmission and the differential. 2. lifting vehicle, check 2 front wheel tyres, no foreign body, chassis without collision trace. Check the bolt torque of sub frame and control arm, no loosening. Check the clearance between the left and right front wheels and the left and right shaft clearance normally. 3. the N gear is hung on the transmission, and the left front wheel is rotated with the hand. After repeated rounds of rotation, it is found that the left driving shaft will make abnormal noises in the course of rotation. It is further determined that the abnormal sound is emitted by the inner ball cage on the connecting side of the transmission shaft and the transmission, and the outer ball cage connected with the wheel is no exception. Turn right front wheel check, normal. 4. remove the left drive shaft, turn and shake the inner and outer cages: the outer cage is normal; there is a sound inside the inside of the cage. It feels like there is no grease inside the inside of the cage, but the dust cover is well sealed and no leakage. Dismantle the upper cover of the cage in the transmission shaft and find that there is no grease inside the inner cage, which causes abnormal noise due to dry grinding. Fault analysis: the car is a new car. The left front drive shaft has lost the grease inside the ball cage when it left the factory, causing dry friction to cause abnormal noise. Troubleshooting: replace the left front axle assembly. Maintenance summary: this case is a typical chassis noise problem, through inspection to determine the specific parts of the abnormal sound of a transmission shaft inside the ball cage, check the ball cage of the dust cover seal is normal, no leakage. After further dismantling, it was found that there was no grease in the inner cage, and the abnormal noise caused by dry grinding was a question of production quality.