2000 Alero A/C knocking

My 2000 Alero V6 3.4L has developed a rapid knocking sound when the A/C compressor is engaged. History: I bought the car used back in 2004 w/ 52Kmiles on it, after about a month the A/C wouldn't cool (car had sat un-used for ~6 months). Back then I used one those A/C re-charge bottles from A-Zone, system cooled right down and worked fine until about a month ago. During a long drive, it just stopped cooling (was blowing warm air). I got another re-charge bottle and added it in; using the low-side port near the firewall, it took the whole bottle, and the gauge on the bottle indicated that the system was fully charged. It is still working (blowing cold air), but now a few weeks later it makes a loud knocking sound, loudest when idling and seems to lessen/go away above 1200rpm. I'm pretty sure it's the compressor since noise goes away when I turn off the A/C. I've worked on cars a lot but never messed with A/C systems; is this noise something I can do something about, or is it a sign that the compressor is dying? Should I get the system flushed/re-charged? Any help with A/C know-how would be greatly appreciated.

The knocking sound is probably the compressor internals or shaft bearing. When you have a system leak you not only do you lose the R134a refrigerant but the PAG150 oil which the compressor needs for lubrication.