Horrible grinding from aircon

[miniman]

Oh dear, the air con on my A4 doesn't sound good. When the engine is idling, and the aircon is running, there's a horrible grinding sound from the engine which I suspect will be the aircon compressor bearing, or pulley, or something. Noise goes away when the climate is switched to econ or switched off altogether. Still blows cold though.

Any ideas on what this will be, and possible repair costs??