The brief
Mr David's S300 had a loud, disturbing knocking from the front, and he was referred to us to find it. A noise like that usually has more than one thing behind it, and on this car it did, so the job was to scan it, inspect it properly, and deal with everything that was contributing rather than chase one symptom. A few things stack up to a front-end knock with vibration: worn spark plugs make the engine misfire and run rough, which shakes; sagging engine mounts stop absorbing that shake, so the engine knocks against its stops and the vibration comes into the cabin; and tired front air struts let the suspension crash through its travel over bumps. Each is a real fault on its own, and together they make exactly the noise this car was making, so all of them needed putting right.
The diagnosis
The Mercedes diagnostic handset pulled the codes, misfiring and an intake air leak, and a proper inspection joined the dots: badly worn spark plugs behind the misfire and the rough running, the engine mounts sagged and cracked so the vibration and knocking came straight through, and the front Airmatic air struts worn out. The rest of each system checked out. That's a spark plug replacement, new engine mounts, and renewed front air struts, the whole picture dealt with, rather than fixing one and leaving the noise.
The work
The badly worn spark plugs were replaced with a fresh genuine Mercedes-spec set, gapped and torqued, the ignition side checked and the intake leak addressed. The sagged engine mounts were replaced with new genuine Mercedes-spec mounts, every fastener torqued to the manual figures and the drivetrain checked sitting square. The worn front Airmatic air struts came off and new genuine Mercedes-spec struts went in, the air system bled, the ride height calibrated, and the misfire and suspension faults cleared. A road test confirmed a steady idle, no misfire, no knocking or vibration, the car sitting level, and a composed ride.
The outcome
A steady idle, no misfire, no knocking from the front, no vibration through the cabin, the car sitting level, and a smooth ride, with no warning lights. David got his S300 back fully restored from the knocking, quiet and smooth. A noise like that is usually a stack of faults, so dealing with the plugs, the mounts and the struts together fixed it properly in one go rather than chasing it back a piece at a time.
