AC question

Ok I Have A 93 Cvpi And The Air Conditioning Works, But What I Think Is The Compressor Is Making Some Noise And Its Pretty Loud. Now I Guess It Runs In A Cycle Because It Makes The Noise Only When The Compressor Clutch Opens. But Now It Actually Runs All The Time So In Turn The Noise Is Constantly There When Ac Is On. Now Is This Suppose To Happen? And This Noise That Im Hearing Does This Mean A Possible Problem On The Brink? Or Is It Just Showing Its Age? Finally What Is All Involved Price Wise If This Compressor Does Go. And What Is Usually The Thing That Goes Wrong With These Units