If it`s only noisey with the aircon switched on your aircon compressor drive pulley may be loose.It`s held on by a small nut and they do have a habit of working loose.The noise only appears when the aircon is on because the centre of the pulley (the loose part) is not turning when the aircon is switched off and the magnetic drive is in free wheel mode.If I were you i`d be heading straight to my garage to get it checked and tightened before too much damage is done as a new aircon compressor costs in the region of ?350 - ?400.
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