DIAGNOSIS No, the panel is not breaking. This is a normal physical characteristic of OLED TVs, including the C3. The sound is caused by the expansion and contraction of materials (mainly the plastic rear cover and metal frame) as they heat up and cool down.
FIX There is no fix because it's not a defect. It's a byproduct of the thin design and thermal management. The sounds are most noticeable during the first 15-30 minutes after turning the TV on (heating phase) or after turning it off (cooling phase).
IF THAT DIDN'T WORK If the sound is loud, repetitive, or sounds like an electrical arc, that's different. Ensure the TV is plugged directly into a wall outlet, not a power strip with surge protection that's failing. Also, make sure all screws on the back panel are snug (but don't over-tighten).
HEADS UP Do not attempt to open the TV to silence these thermal noises. You will void the warranty and risk damaging the extremely fragile OLED panel. If the sound is accompanied by a burning smell, picture issues, or the TV shutting off, then contact LG support immediately.