Are you familiar with SLS? You might have a dead accumulator on the side that clunks.

I don't think you can swap in lowering springs and expect a healthy suspension. SLS is calibrated for ride height and will do what it can to achieve it. You can adjust ride height but that will likely lead to uneven tire wear. A spring meant to take dynamic weight will cause comfort issues with SLS. The coil spring there now only takes static weight. It holds up the car when the SLS strut is inactive or being serviced. It won't support the weight of the car loaded and being driven.

If you insist on lowering the car, get an S320 or S420 without SLS or see about retrofitting a non SLS rear suspension. Mind the diff ratio.

Sixto
98 E320s sedan and wagon