I've had one that's existed for as long as I remember, but it randomly became a lot more prominent when I was around 16. I don't notice it most of the time but occasionally I'll accidentally focus on it and then it'll be hours before I forget about it. Can be very annoying.
I also have a couple of smaller ones but they're so faint I don't really know them as, uh, "intimately" as the main one. :P