Your camera and the head unit has a bit different frequency, or the same but with a phase shift. The cameras' sensor is not having a continuous picture, but a screenshot 50-60 times per minute, so sometimes it can record a full image from the screen, sometimes it gets a half-updated/not updated screen. That's causing the flickering. On more expensive cameras you can sync it to the screen frequency.

I think it is the touch screen that activates.

Thanks will try it out. How did you find out about it?