The compressor does make a slight humming noise uder normal operation, as stated before, it is difficult to tell if it is normal or not. If everything is working fine, I would not be too quick to throw parts at the car.
You are correct, it uses r-12, unless it has been converted.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record