Diagnosis
That's electronic noise, likely coming from the amplifier stage inside the soundbar. It's often more noticeable with certain inputs or volume levels.
How to Fix It
First, cycle through the different input sources (use the 'Input' button on the remote). Does the hiss disappear on one input (like Bluetooth) but not another (like Optical)? If so, the issue is with the noisy source or the cable feeding it. For example, if the hiss is only on the optical input, try a different optical cable. Also, lower the volume on the source device (your TV) and raise the volume on the soundbar to see if that reduces the noise floor.
If That Didn't Work
Check your power source. Plug the soundbar directly into a wall outlet, not a power strip shared with other noisy electronics like a router, TV, or game console. This kind of hiss can sometimes be caused by 'dirty' power or ground loops introduced through other connected devices.
Heads Up
A very faint hiss is sometimes normal with class-D amplifiers (which this uses) when you put your ear right up to the speaker. If it's audible from your listening position at normal volumes, that's a problem. If it's new and wasn't there before, it could indicate a component starting to fail.