Diagnosis
Analog circuit noise. This is a common trait in this soundbar's amplification stage when processing a sudden change in signal level or a digital source switch.
How to Fix It
First, rule out source device issues. Try a different audio source (e.g., Blu-ray player instead of TV ARC). Use a different input port on the soundbar (Optical instead of HDMI). If the noise disappears, the issue is with the original source device's audio output.
If That Didn't Work
Update the soundbar firmware. A firmware update can refine the digital-to-analog converter (DAC) processing and reduce circuit noise. Download the latest firmware for S40Q from LG's website and update via USB.
Heads Up
This model does not have a dedicated 'AUTO VOLUME' or 'Night Mode' feature. Sudden large jumps in volume from source material (like commercials) will produce a noticeable amplification click. Manually manage volume levels from your source device.