The air con on my 1999 406 Hdi has started to make and rattling sound which increases and decreases with engine speed. At first I thought is was the water pump, however if I switch off the air con I can hear what I believe is the compressor shut down and then it stops. The sound is not present if the fan is operated without the air con. Has anyone encountered this problem? Does it mean a new compressor? Is it expensive? Thanks