I've been getting a cyclic knocking from the engine (I'm assuming its constant although I can only hear it outside the car at idle). It doesn't stop after the engine has run for a while. Only stops when I turn the air con off, starts after a 1 or 2 second delay from turning the air con back on.

I'm assuming its the bearings in the compressor, can they be changed seperately or do I need a new compressor? Can they be changed without degassing the system?

Maate,
if it's the A/C compressor bearings then you'll need to take it to an aircon place to have the compressor removed/replaced with either a new/recond/or 2nd hand compressor. The system should also be serviced at the same time ie: receiver drier changed, condenser air side cleaned, compressor oil changed and refrigerant system tested for leaks.

Cheers

This is a long shot but I'd also check the bolts that hold the compressor to the engine are tight - if they are loose they will allow the compressor to move. If not then it will probably be a compressor problem...usually they are throw-away jobs and it's cheaper/quicker to fit either a new or 2nd hand unit. They are not serviceable without de-gassing unless it's the clutch bearing. If you want to save some money you could change the compressor yourself as long as you do some research into what is involved - you probably would also need to change the drier at the same time and use new "O" rings. Then take it to your aircon man to regas. New compressors are on Ebay for about $400 and 2nd hand for $50 up.
If it was me, I would drive it till it gets worse and then attend to it...it may not even get worse - winter is almost here so you have some breathing space - but that ain't a recommendation.....just what I'd do.
Are you sure it is coming from the compressor and not the aircon fan in front of the radiator that also comes on with the compressor?

bit late now, but in feb? U-Pull-It here was selling air con compressors for $11 ea. to VIP members.