We had the same problem in our 2 years old TV. What we did is disconnected one connect box and connect it again, the line is gone and hoping that it will never show again.
Techs came out at first they said they would have to replace it. But was advised when they called in the ticket due to it still being within the 1yr warranty they would have to replace the whole back panel.
I've had this problem too. I shelled out 2500 for tv year ago and basically they've done nothing multiple emails and phone calls with tech support they won't say anything except troubleshooting. Stick with LG is the lesson I've learned. Clearly it's a problem they are familiar with many of their TVs are experiencing it but won't even try to help and no tv repair in my area.