Hi, my name isXXXXX 500SL uses hydraulic valve lifters. The lifters are adjusted at the factory and do not require adjustment unless the cylinder head is disassembled for a valve job. Adjustment is made using a special tool to set the center point of the lifter travel. The adjustment is changed by replacing the shim that sits on top of the valve that the rocker arm rests on. If the valves are noisy most likely either a lifter is bad or the camshaft or rocker arm is worn.