1998 E320 A/C compressor claking sound
 

occurs when not engaged, I hear a claking noice from I believe the clutch assy on front of unit. There are 3 screws that hold the front portion on. Screws seem tight

I know you say it sounds like the compressor, but occasionally the belt tightener can make a noise and it will sound like it's coming from another component. The noise can change when the drive load changes, such as when the compressor cuts in/out. For some reason I thought the compressor was always engaged on this car, maybe they started that with the 2000 S Class though.......
Gilly