The original oil in your compressor is not miscible with R134a; you probably have oil return problems in turn causing noise.
Change the oil for a Polyol Ester oil; by the way one change is not sufficient, I would recommend 3 changes to ensure you have sufficiently removed the mineral oil.
Again using a drop-in such as Isceon 49 avoids this, as well as removing the chance that such things as the shaft seal of the compressor may fail due to incompatibility with the new oil or refrigerant.
There are several different "retrofit" refrigerants to replace R12, in my opinion Isceon 49 has the closest operating characteristics to those of R12 and does not require any modification of the system other than a replacement filter drier (which you will be doing anyway). By the way no oil change needed.
The mail washer program is written by a guy here in Christchurch; he even includes his phone number on the program, so I believe he is probably pretty sincere. He provides a free basic (almost full) version and basicly asks that if you like that you purchase the pro version which has a few extra features.