I'm not sure abt the air con not cold but i had the same hissing sound problem. Got very loud these few days and decided to go for maintenance. Verdict - Fan belt cracked, according to the mechanic latio's fan belt will surely cracked at 50k. Changed mine and sound appears to gone. Will monitor status.
Damage - $67.50 with autosaver discount for original fan belt b4 gst
Labour - $25 installation b4 gst