2000 New Beetle AC "Knocking" Sound

I have yet another AC mystery that I could use help with. The car has started making a knocking sound when the AC is turned on. The knocking will increase in frequency along with engine RPM, and is most noticable at lower RPM. When the AC is off, no knocking. Other than that, the AC still blows quite cold and operates well. I had a similar issue last year, which wound up being a problem with a leak and no refrigerant in the system which caused the compressor to go on and off rapidly, however, this is different. Any thoughts? Thanks in advance!

I have yet another AC mystery that I could use help with. The car has started making a knocking sound when the AC is turned on. The knocking will increase in frequency along with engine RPM, and is most noticable at lower RPM. When the AC is off, no knocking. Other than that, the AC still blows quite cold and operates well. I had a similar issue last year, which wound up being a problem with a leak and no refrigerant in the system which caused the compressor to go on and off rapidly, however, this is different. Any thoughts? Thanks in advance!

Check the drive belt as it may be faulty and also check the bolts that hold the compressor to the engine as they could be loose.