High-pitched whistling or hissing noise from the unit while running.

Safety first: Do not attempt to open the refrigerant circuit yourself.

• Differentiate sounds: Mild hissing or gurgling can be normal. Sudden loud hissing when the unit is off may indicate a leak and should be investigated by a technician.
• Check panels and gaskets: Ensure all access panels are properly installed and gaskets are intact to reduce wind noise.
• Monitor performance: If heating performance is normal and no error codes appear, the noise may be characteristic of the expansion process.
• Call technician if abnormal: If noise is new, very loud, or accompanied by poor performance, have a professional check the expansion valve and refrigerant circuit.