Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
A bit of scratched up grayness. Always good.
Source Dmitry
It’s like Shine Dotted’s sister, only rotated 45 degrees.
Source mediumidee
Might not be super subtle, but quite original in its form.
Source Alex Smith
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
A repeating gloomy background image. This one consists of a pattern of black chains layered on top of a dark textured background.
Source V. Hartikainen
A pattern derived from part of a fractal rendering in Paint.net.
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Sharp diamond pattern. A small 24x18px tile.
Source Tom Neal
Light gray pattern with an almost wall tile-like appearance.
Source Markus Tinner
From a design in 'Storia del Palazzo Vecchio in Firenze', Aurelio Gotti, 1889.
Source Firkin
A seamless pattern formed from miutopia's cakes on a tablecloth.
Source Firkin
Pattern #100! A black classic knit-looking pattern.
Source Factorio.us Collective
A smooth mid-tone gray, or low contrast if you will, linen pattern.
Source Jordan Pittman
Smooth Polaroid pattern with a light blue tint.
Source Daniel Beaton
Derived from a drawing in 'Historiske Afhandlinger', Adolf Jorgensen, 1898.
Source Firkin
Sharp but soft triangles in light shades of gray.
Source Pixeden
Prismatic Snowflakes Pattern 2 No Background
Source GDJ
Used in small doses, this could be a nice subtle pattern. Used on a large surface, it’s dirty!
Source Paul Reulat
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin