306 air con ?

hi i looking for help with my air con in my 306 its2001 1.9 diesel .the air con was not working wen i got the car' the light would turn on but that was all so i got the a c filled with gas and it was fine for the last two weeks or so but now wen i turn the a c on iget a loud hissing noise from in side the car comeing from the L H S the a c is still working fine but not sure what the noise is cant see any thing leeking thank for the help