Diagnosis
This is amplifier noise. It's most common with the optical input or when the volume is high with no signal. A ground loop or poor-quality source cable can cause it.
How to Fix It
First, change the audio input. Use the remote to switch from 'D.IN' (optical) to 'HDMI'. If the hiss stops, the issue is with your optical cable or TV's optical output.
If That Didn't Work
Perform a factory reset. With the soundbar on, press and hold the 'Play/Pause' button on the soundbar itself for about 10 seconds until 'INIT' appears. This clears any corrupted settings.
Heads Up
Do not use a generic power cable. Only use the original Samsung adapter. A mismatched power supply can introduce significant electrical noise into the system.