A/C not getting cold and making a knocking noise?

My wife stated the A/C on her car wasn't getting as cold. I hook gauges to it and took about 120z. Of 134-A to bring the gauges to desired pressures and the ac back to cool, however after allowing it to run I raised the RPMs a bit and as the RPMs came back down I noticed a knock very similar to a rod knock, again each time I revved the engine when the RPMs came down there was the knock [was not knocking prior] so I decided to turn off A/C and the knock went away, I check all mounting bolts [all were tight] I get the a/c compressor is culprit, I just never heard a compressor knock like that.