I get slight headaches when there's certain kinds of flicker. I think if there was no flicker but instead the light itself was too aggressive, it'd cause other symptoms like burning eyes, etc. If it's the same for you there may be some flicker left.
Again, turn any feature off that alters the original image. Explore the settings and make sure you have checked and understood them all. You need to start with a neutral picture. You need to have patience for this. Some websites like rtings explain some of the advanced settings. Take your time to study them.
If you really think it could be the sound, you could try using headphones or just watch without sound for a while.
Without flicker detection equipment, I don't think I would have found the settings that make my current TV usable. I would never have known which exact brightness values have the least amount of flicker, for example. Never have found out that switching inputs or resolutions cause a short burst of 20 kHz flicker, ultimately cause those weird small headaches. I doubt you would see 20 kHz on an OLED though. But you get the point. There's settings and behavior one would not expect to introduce flicker.
You could try to further experiment with the brightness setting.