2000 Century Class knocking noise with AC on

As the title says the truck is a 2000 Century Class with an N14. After a couple weeks of wondering what the heck the loud knocking noise coming from the front of the engine was it finally hit me that it was the AC Compressor. When the Compressor is on there is a loud knocking noise. Soon as I turn it off it goes away. It is only noticeable to me when I get out of the truck with it idling. The thing blows freezing cold air so it is not like it dried up and is chewing up its own bearings.

Are these noisy compressors or do I have a bearing going out somewhere?

If the latter is the case is there a common failure point on these ie. a bearing in the compressor or in a pulley?

Thanks