Scoo

I was wondering if anyone can help me. I've got a strange problem which appears to be aircon related, but the story may seem a bit wandering...

My wife has a new Fiat Panda which gets a weird metallic ringing noise which the dealer has never been able to find (bollox in my opinion). When I ordered my Scoob, the dealer lent me a GX Sport for a couple of weeks as the clutch had died on my part-ex (got price without showing them the car - bonus!), but this car had the same metallic ringing, which I found stopped if you clicked the air con off on.

I don't know if this is the case on the Fiat as we haven't driven it, preferring to pile the mileage on the freebie car instead

I got my new MY05 WRX PPP today, and again there is this metallic ringing noise when the aircon is on! WTF is it, and how can I stop it. I have to assume that it's the same problem on all three cars, but is it normal?

Thanks in advance for your help!