The brief
The A4's owner came in with an unusual sound from the front, the kind of clunk or knock that tells you something up there has gone slack. He brought it in. The culprit was a front shock absorber losing pressure. The strut damps the spring, it stops the car carrying on bouncing after a bump and keeps the tyre pressed on the road through corners and over rough surfaces, and inside it the oil and gas charge does that damping. When the seal wears the strut bleeds its pressure, so it stops damping properly, the body floats and bounces, and the noise comes from the strut moving against worn parts. A strut that's lost its pressure doesn't recover, and you do the pair so both fronts behave the same, so it needs changing.
The diagnosis
On the lift each front strut got a bounce-and-inspect. One had clearly lost its pressure, weeping and barely damping, and the other was tired alongside it from the same mileage, the body carrying on after a push instead of settling in one. The rears were still doing their job. When the pair on an axle have gone, you do them as a set with fresh top mounts, a fresh strut next to a tired one gives you a car that handles differently side to side, so the call was both fronts.
The work
Both front struts were unbolted and removed, and a new genuine Audi-spec pair fitted with fresh top mounts and bearings, every fastener torqued to the manual figures. With both sides back together, the front geometry was checked and set so the new struts weren't fighting a misaligned corner. A road test confirmed the noise was gone, the ride had settled, the front planted again.
The outcome
A composed ride that settles in one motion, flat and stable through corners, the noise gone, and the front planted again over bumps. The A4 went home riding properly again. A strut that's lost its pressure only gets worse and they take ride quality and grip down with them, so doing the pair as a set with fresh mounts reset the front end rather than leaving one side lagging the other.