A/C Compressor Noise

I have a 1997 Chevy Silverado K1500, with 5.7L engine, automatic trans and about 110K miles. Recently the A/C compressor began making a chirping noise when the engine is idling. The noise goes away with any increase in engine speed. When I shut the engine off there is a whistling noise for a couple of seconds. Pressing the button to turn of the A/C compressor stops the chirping noise, so I am fairly certain it is from the A/C. The system cools fine. As for past repairs, last October the A/C stopped cooling due to refrigerant leak. The ports where the refrigerant lines connect to the top of the compressor were replaced and the system recharged by a local A/C repair shop. What is causing the noise and will it lead to further problems?