A/C compressor makes noise

I have a 74 Duster /6 with factory A/C. Running R134A. Compressor is known good. Running the correct pag oil and the correct amount of A/C charge. System blows cold air, but the compressor is very noisy ( snapping, chattering, especially at idle ). Sounds like it may be coming from the valve that sits in the compressor suction side port. I have no idea what this valve does or what it is called. Looks like it may be adjustable. When I replaced the compressor with a new reman, I just switched this valve over to the new compressor. Is this valve the problem and what is it - what does it do? Thanks, John