Rattling noise with air-con on

I have a rattling/ticking sound when the air-con clutch engages.

I have experienced a few (2 or 3) failures of the air-con system in the last 6 months (i.e. very rare) where the entire system has refused to work. I had the system re-gassed but it has done it once since also.

I pulled off the clutch plate and cleaned it up, have not down-sized the shim but anyway this is just back story in case it might be relevant. The bits I can inspect from that job seem to be in good condition.

When the air-con clutch engages there is a loud rattling akin to a loose belt tensioner (which was replaced on its own account recently, it isn't that). Other than the noise it seems to work fine 99% of the time, bar the exceptions above.

Because the noise goes when the clutch disengages, I reason it is not the pulley itself which continues to spin. If it is the clutch, my next question is: can the clutch be replaced separate to the compressor?

Other thoughts as to cause before I take it to Ford?