[Novice07]

Hi, I've just bought a 2nd hand Fiesta Zetec build date Sep 04 (private sale). I had it checked out by the RACQ before i bought it and also took it for a test drive and didn't hear any problmes with the air con. I've been driving it around now for over a week and i've been getting these funny periodic hissing noises from the dash vents when i drive with the air con on.... also when i turn the car off and the air con was on there is a very long and loud hissing sort of noise that goes on for about 30 - 45seconds when the car has been turned off. Does anyone else find they have this problem? or could this be a sign that my air con is on the way out?

Thanks for any advice