AC not working

Hello all, I've recently purchased a 2002 mini cooper, everything works absolutely great with no problems... all but the AC. As summer is coming up it's something that I would like to fix by then, I've checked the compressor clutch and that's working fine, the compressor spins and stops via the button in the cab fine, makes a little bit of a repetitive hiss noise inside the cab however (also sometimes almost like sticks hitting a fan, but on and off)and externally as well. the refrigerant appears to be full, as I have checked the valves on the side and fuses are all good, the cabin air filter has also been changed, would anybody have any other suggestions as to what could be the problem? Thanks for the help, can't find an issue similar online.
Owen