The brief
The A6 had a knocking sound when turning, left and right, the kind of clicking knock that comes through under steering load. That points at a worn drive shaft. He brought it in. The drive shafts carry the power from the gearbox to the wheels, with constant-velocity joints at each end that let the shaft turn while the suspension moves and the wheels steer. Those joints are packed with grease inside a rubber boot, and over the miles, especially if a boot has split and let grit in, the joint wears, develops play, and clicks or knocks under load, worst on full lock when the joint's working hardest. A worn CV joint doesn't recover, and once a joint's that worn the practical fix is a complete drive shaft, so it needs replacing on the affected side.
The diagnosis
On the lift the drive shafts were checked, the joints felt for play and the boots inspected. The drive shaft on one side had a worn outer CV joint with play you could feel, which is exactly what makes that knock when turning. The other shaft and the rest of the drivetrain checked out. That's a drive shaft replacement on the affected side, a complete shaft with fresh joints and boots, rather than trying to rebuild a worn joint.
The work
The affected drive shaft was unbolted from the hub and the gearbox and removed, and a new genuine Audi-spec drive shaft fitted, the joints greased and the boots checked, every fastener torqued to the manual figures, with a fresh hub nut. The other shaft's boots were given a check while everything was accessible. A road test confirmed no knock when turning, left or right, the drivetrain quiet under load.
The outcome
No more knock when turning, the drivetrain quiet on full lock, and the power going to the wheel cleanly. The A6 went home with the drive shaft sorted. A worn CV joint only clicks louder until it fails entirely, so changing the shaft on the affected side put it right before it became a being-stranded problem.
