The very short sharp 'crack' impulse noise I've some times heard is emphatically electronically generated albeit probably mechanical in origin (sound actuators bonded to the back of the actual screen). It is not predictable nor repetitive but has always been at night and whilst the TV is actively powered up being used.
Weeks separate those few instances thus far experienced. Certainly not multiple instances per viewing session. Is your stuff anywhere near to a cooling or heating draft from central air conditioning or heating?
My kit is situated quite near to a sunward-facing window (though with a netting diffuser). Strong morning sunshine does hit that rubbish-quality back panel thing covering what is some what optimistically also known as the woofer but no actual draughts. The window is difficult to reach (TV is in the way) and is never opened. If the TV rear assembly were to be making temperature based mechanical noises it would surely be then but this has not been experienced.
Further... my stuff is 'perched' on a small but massively constructed solid wood 'coffee table' of a size exactly equal to the footprint of the TV's base. Solid cement & tiled ground floor. IOW there is absolutely no flexing or vibration transmitted into the TV. If yours is directly floor mounted (eg using the whole width of the screen) and it were to be on a wooden or otherwise sprung floor then the TV might being micro-flexed by diverse movements of either your family or even by people walking up and down common staircases outside. An out-of-balanced loaded washing machine can do similarly. Heavy road traffic or underground railways also emit low frequency vibrations. Check your sound equaliser settings too as the lefthand side (base) sliders might have been elevated. Your movie might have some embedded very low base that is at the root of your cracking noise (eg the woofer poorly attempting infra-base); especially if the overall volume is set loud.
Good luck sleuthing:-)
