Diagnosis
That's the plastic housing expanding and contracting from heat changes. The mounting clips or screws are too tight.
How to Fix It
Unplug the TV. Lay it screen-down on a soft blanket. Remove the back cover. Check all screws along the perimeter. Loosen each one a quarter-turn, do not remove them. This allows the plastic to flex without popping.
If That Didn't Work
The sound may be coming from the power supply board. A failing capacitor can cause a rhythmic tick. Inspect the PSU board for bulging tops or leaking electrolyte.
Heads Up
Do not overtighten screws when reassembling. Snug is enough. Cracked plastic is not repairable.