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.