Whistle from fan

Hi All,

Noticed for a while that I've got a whistling interior fan, however doing some google searches have highlighted it could be a hole within the fan casing.

But... It only happens when the air con is switched on (and fans), turn the air con off but leave the fans on and it stops. So is there an additional fan/routing for the air con feed that only comes into play when the air con is switched on?

Cheers

Is it a hissing sound when the AC is on? is it continuous? Or come and go in time with the AC clutch? If the later it's probably the evaporater in the heater unit?? they can be noisy sometimes!!