Diagnosis
That's the sound of the TV's plastic casing expanding and contracting slightly due to heat changes from the internal components. It's common, especially in the first few weeks of use or during long viewing sessions.
How to Fix It
This is typically a normal physical characteristic, not a malfunction. However, you can minimize it by ensuring the TV has proper ventilation. Check that all the vents on the back and sides are completely clear—don't have it flush against the wall or crammed into an entertainment center. Leave at least 2 inches of space on all sides. Also, go to Settings > General > Energy Saving and turn OFF "Auto Power Off" and "Screen Off." Sometimes the clicking coincides with these power-saving features engaging and disengaging components.
If That Didn't Work
If the clicks are loud, rapid, or sound like they're coming from inside near the power supply, it could be a failing component. As a diagnostic, switch the TV's power mode: go to All Settings > General > System > Additional Settings > Quick Start and turn it OFF. This changes how the TV manages power. If the clicking changes or stops, it's likely related to the power circuit and you may want to contact LG support.
Heads Up
The clicks often lessen over time as the materials settle. If the TV works perfectly otherwise—no flickering, no failure to turn on—it's almost certainly just thermal expansion and nothing to worry about.