Your expansion valve (part of the aircon system) is the one making the hizzing sound, this may be due to freon gas leak which then lowers the gas level and possibly you would need to have your aircon system service and repair, like change of drier tank, expansion valve, general cleaning, thats all there is, if the hizing sound persist after the general cleaning, you can ligitimately complain to the supplier of the expansion valve, not all of this part are made with standard quality, there are chances that you will meet up with a defective valve even if its new.