Air con making wining sound !!

[treeman]

When I switch my air con on, a wining sound comes from behind the module containing the heater controls. It sounds as if a little fan has got worn out bearings or something like that. It's only with the air con on, with just the normal fan there is no noise.
Does this sound like the symptom of an imminent failure of something in the air con? I have had the air con coolant checked and the pressure in the system is ok.
Also after switching the car off, I can often hear a quiet ticking sound coming from the same area - this has been like it for a few years but so quiet it never really bothered me.