Diagnosis
That hiss is likely ground loop noise or electrical interference coming through the optical cable. It's surprisingly common, even though optical is supposed to be immune.
How to Fix It
First, unplug the optical cable from both the bar and the TV. Give it a good visual check for any deep scratches or cracks on the cable itself. Then, plug it back in firmly at both ends, making sure it clicks into place. If the hiss is still there, try a different optical cable if you have one. Cheap or damaged cables are a frequent culprit.
If That Didn't Work
Try plugging the JBL Bar's power cord into a different wall outlet or power strip than your TV. Sometimes power supplies from different devices sharing an outlet can introduce noise into the system, even over optical.
Heads Up
If you switch to using the HDMI ARC connection (which also carries audio), you'll bypass the optical cable entirely and this hiss should disappear, plus you'll be able to control the bar's volume with your TV remote.