the gas in the zircon system is running low. you will find in the coming weeks that it becomes less efficient and not a cold.
The gas passes through an expansion valve as it goes into the car, when the gas is low the is not so much of it to expand and the pressure is lower on the expanded gas side of the valve, this means that as the air con cycles on and off (as it is not constantly on) the gas will hiss as the pressure equalises on both sides of the valve, with a healthy and recharged system there would still be a relatively high pressure on the other side of the valve. In a healthy system it would be about 200psi on the high pressure side of the valve dropping to about 50psi on the lower pressure side, however if there is low gas then there will still be likely about 200 ish psi on the high pressure side but it can drop much lower on the low pressure side, this allows the hissing noise to take place.
So short answer, you need the Aircon recharging.