Combined pass has noise that's not present in the any of the render passes

I’m rendering a flat-colored texture without any lighting (texture -> surface), but somehow the output image has tons of noise. The Diffuse Color pass has no noise what-so-ever, but the Combined pass is noisy.

The noise on the output image is a constant pattern. It’s the same whether I use EEVEE or Cycles, 1 sample or 500. Even moving the camera doesn’t change the noise pattern or intensity. It seems like a simple overlay, but I can’t find where to turn it off. It causes the video encoder to go crazy and generate ugly encoding artifacts all over the video unless I use a ridiculously high bitrate.

Here’s a picture of the noise in question.

Does anyone have any way of getting rid of it?

Please post a .blend file … and tell us exactly which Blender version you are using. (The complete version-string from the “About” box.)

Just for giggles, I updated to 2.91.0 and now the noise is completely gone in the final combined render. I didn’t change anything in the file, just opened it in the new version and hit F12 and it was perfect.

It seems that the noise is coming from rounding errors during the export process. When rendering images out at 8 bit depth png, the noise is present in the output file but not the internal Blender render viewer. By switching to 16 bit depth png output, the rendered files came out perfectly noiseless.

Because of this error, a 1920x1080px 8-bit depth PNG of a pure green card took up 2.1 MB while the 16-bit depth PNG used only 56 kB.