I have a 1998 E300 D and have noticed when I engage the A/C Compressor I hear what sounds like a rat a tat tat or a fan contacting plastic shroud. I checked....the fan is not hitting the shroud. The belt is not loose. After a closer look.....it appears the noise is coming out of the back of the compressor so I believe whatever I am hearing is coming from the internals of the compressor.....ANY HINTS?

 

Does your AC work?
Is the system correctly filled with refrigerant?
There is a speed sensor on the back of some compressors. Possible that this is culprit but unlikely.
The clutch on the front of the compressor also makes a "Clack" type noise when it engages. Maybe the clutch is engaging then disengaging repeatedly?
However, I suspect this is a new/recon compressor job and that an internal component has failed. If so you need to be careful the compressor doesn't lock up and potentially take your serpentine belt with it (although the speed sensor on the back of the compressor is designed to prevent this!
Stuart