A hissing noise coming from your HVAC system is not something to be ignored. Among the possible causes are leaky refrigerant lines, an internal valve leak or a serious issue with your compressor.
If a refrigerant leak develops, you may hear a light hissing sound as the refrigerant leaves through the leak. This kind of leak can also be accompanied by a bubbling sound.
If the issue is a valve leak, it should be repaired by a professional as soon as possible. The job of the valves is to help with the pressurization of the refrigerant gas and left unrepaired, the leak will worsen.
If it turns out that the compressor is the issue, this could be a serious situation because it is an indication that the pressure inside the compressor is too high. If you hear hissing, or shrieking, coming from your compressor, turn off your air conditioner immediately and call an expert technician.