BMW Repair: BMW 323i 99 Compressor, 323i bmw, metal debris

Question
I purchased a 99 323i BMW like 2 years ago, at the end of the last summer I noticed a noise in the AC compressor (a tapping/chatter) It's very pronounced at idle speeds and seems to go away at higher RPM's. If I turn the AC off, the car purrs quietly. Since it was close to the change of seasons I did not check it. This year I had the compressor changed but the new one started having the same problem.
What needs to be done to get read of that noise? And would it lower the performance of the vehicle? Some researches shown that it could be caused by some metal particles inside the AC system. In that case, what is the best and less expensive course of action?  
Your suggestions are greatly appreciated,

Thanks,


Answer
Arturo. Try Evacuating the system, replace the reciever/dryer canister and any O-rings also replace the suction pipe from expansion valve to compressor with updated part 64 53 6 933 368. Use only PAG oil. Use an ounce of oil for the reciever/dryer and half ounce for the line. Make sure not to use A/C refrigerant with sealant.
Fill to capacity to green under hood label. Look for any signs of metal debris.
-Evan