Air con - odd "grinding" noise
Apologies if this has been covered elsewhere, but another item on the snag list of my new 159tbi sportwagon. When the air con is engaged, there is an odd hollow grinding noise that changes in pitch as the engine revs. It isn't present when the air con is switched off, so I'm worried the compressor has given up the ghost. It still seems to blow reasonably cold when on however. Is this something as straightforward as needing a service and re-gas, or is it likely that the compressor has developed a problem?
Thanks in advance for any tips!