Random Shadows on LG TV

Hi I have an old LG 42" TV (2013 model).

Recently, I noticed that in one specific area of the screen (top close to the left edge of the screen) there is a random vibrating shadow. It is not always there and it is not related to any external device or cable as I checked them and it happens even when I am seeign TV (and not a movie from the DVD player or USB). I changed the setting from ECO to normal and it happened again.

As said, it is completely random. I may not see it for hours.

Any idea that what is it and how serious is that?