Indoor unit produces a high-pitched whistling or hissing noise during operation.

Steps:

• Check louvers: Adjust horizontal and vertical louvers to a more open position to reduce air velocity.
• Clean filters: Dirty filters increase air velocity and noise; clean them thoroughly.
• Fan speed: Reduce fan speed from High to Medium or Low and see if noise decreases.
• Normal operation: Mild hissing during mode changes or defrost is often normal refrigerant flow noise. If the noise is very loud or accompanied by performance issues, have a technician check system pressures.