sounds like a lifter
 

my 88 420sel definitely has a noisy lifter on the passenger side. how hard is it to replace these on this type engine. or could it be something else. but i'm sure it's a noisy lifter sound. also, at 80 mph the car has a thud vibration similar to an unbalanced tire but nothing comes thru the steering wheel which i know doesn't eliminate the rear. the engine really stumbles at an idle, not much power and again at 80 the vibration is terrible. could an engine cylinder not working properly due to a stuck lifter cause a rough running engine. i just bought my 2 mb last month and just ordered the cd roms on both and bought what books i could find. any help or suggestion on the lifter noise would be much appreciated. tks much