The brief
The S350 had a knocking sound from the front over bumps and uneven roads, the kind of noise that nags and is easy to chase up the wrong tree. Having a good workshop to troubleshoot it properly saves the time and money you'd spend changing the wrong parts. He brought it in and the team put it on the lift. The knock traced to worn front suspension: the lower control arm bushings cracked and the anti-roll bar drop links worn at their ball joints. The control arms locate the bottom of the front suspension and the drop links connect the anti-roll bar to it, and as those joints and bushings wear there's play, so the suspension knocks against itself over bumps. Worn joints don't recover, so the affected parts needed replacing, with the alignment reset afterwards.
The diagnosis
On the lift the front suspension got a thorough check, every joint pry-tested. The lower control arm bushings on both sides were cracked with play under load, and the anti-roll bar drop links had worn ball joints, exactly what makes that knock over bumps. The shocks and the rest of the front checked out. So it was a front lower control arm pair plus a drop link pair, complete parts rather than just bushings, then a four-wheel alignment.
The work
Both front lower control arms were unbolted and removed, and new genuine Mercedes-spec arms fitted with fresh bushings and ball joints, then both anti-roll bar drop links replaced with a new genuine pair, every fastener torqued to the manual figures. The car went on the alignment rig and the front geometry set to specification so the steering tracks true and the tyres wear evenly. A road test confirmed the knock was gone, the front end quiet and tight over bumps and through corners.
The outcome
No more knocking sound from the front, the suspension joints tight, the car flat and composed through corners, and the alignment set so the front tyres wear evenly. The S350 went home with the front end sorted. Worn control arm bushings and drop links only knock louder and the play grows, so doing the affected parts as pairs and resetting the alignment put the front suspension back where it should be.
