The hissing sound of an air conditioning unit is one of the most common indicators of a refrigerant leak. When the AC is running but not cooling, it could be a sign that there is an insufficient amount of refrigerant flowing through the evaporator coils. This can cause the condensation in the coils to freeze, leading to a total system failure and costly repairs. If you hear a hissing coming from the outdoor unit, it could be an indication of a refrigerant leak. Another telltale sign is when the evaporator coils become frozen.
Usually, the coolant in your system provides adequate heat to this coil, which should prevent freezing problems. It's important to check the evaporator periodically so you can spot when frozen crystals form. If you notice any of these signs, it's best to contact a professional HVAC technician to inspect your system and determine if there is a refrigerant leak.