Clicking Air conditioning noise - getting worse

[treeman]

On my 2003 E53 3.0d there is a clicking noise when the air con is switched on. It goes off as soon as you switch the a/c off. It started as a very quiet click but has got gradually louder over time (6 months or so). It is hardly noticeable at the lowest fan setting but speeds up and gets louder as the fan speed increases.

I originally thought maybe a leaf or something in the fan itself but there is not clicking at all when just the fan running - only when the a/c is on - any ideas where the problem may lie?

I've searched this forum any some others but can't track down any similar issues.

Thanks in advance.