since the car has been run so far without filters so evaporator will be dirty which will impact the blow of fan also. I suggest go for a full ac service from abu-bakar Universal car AC G8 islamabad. Get the filters installed as well.
Another import thing is that when you get evaporator serviced, there is a thin lining of foam which acts as a sealant when our local mechanics use pressure water to clean it which removes the thin layer. Make sure that he installs a new foam layer after service otherwise outside air will come inside even if car is set to recirculate.
If hissing sound is constant, then it can be due to dirty evaporator or blower fan. If it comes only when AC is started, then its due to expansion valve (doesn't need changing if cooling is fine)