Does anyone else have noisy a/c?

I have noticed a hissing sound coming from the vents, but it stops when I switch the a/c off.

 

Yes I can always hear the fans running but with a/c on there is this hissing noise too.

Mine doesn't - the only time I've known a car with hissing a/c was our old Polo that needed the a/c serviced and regassed...