Our '98 Passat 1.8T estate (UK registered)on tickover is nice and
quiet when the aircon is off (ideally by setting the heater fan to
OFF), but when the air con is switched on, there is a mechanical noise
which is a lot louder than the tickover.  Turn fan off, noise goes
away, turn fan on it returns, but it's definitely not the heater fan
itself (I used to think it was but it's not!).  Our mechanic diagnosed
a faulty aircon compressor, which with the poor performance of the
aircon last summer seemed to fit.  So we now have a nice new
compressor.
Problem is, the noise is still there, appearing in exactly the same
way.
Does anyone have any similar experiences and so what is the problem
likely to be?  I'd be very grateful for any help!