All renders have slight graininess (both Eevee and Cycles)
I also noticed this strange pattern in my renders and had no idea where it came from or how to get rid of it. Turns out it was
render.dither_intensity, thanks. 
Blender applies dithering to the imageby default to reduce banding. This may be the issue you're experiencing.
To turn off dithering:
- Open the Properties Editor.
- Go to the…
All renders have slight graininess (both Eevee and Cycles)