Help! Very thin horizontal line on my CRT screen

Hey! It’s me again. I have a Sony Trinitron KV-M1400B, and I just noticed a very long, extremely thin horizontal line running across the screen, sitting on top of the pixels.

It’s so thin I can’t even capture it on a photo. From up close it’s barely noticeable, but from a distance it becomes more visible.

Does anyone have an explanation for this? It’s located around the lower third of the screen. The line is way thinner than a pixel—honestly thinner than a hair.

I’m pretty new to this hobby and only got this TV a week ago. It’s totally possible I simply didn’t notice it before and now I’m hyper-focusing on it.

Any ideas what this could be?