I've found a noise in my image, though (ideally) it should be perfectly clean.
So I made an experiment. (Blender version 2.90.1) New file, I set the render size 40 x 40 and changed "View Transform" from Filmic to Standard.
In Compositing tab, I connected RGB to Composite directly. The colour is (R,G,B = 0.40724).
After rendering, I can see (Barely Noticeable) noise in rendered image. But every pixel's colour displayed at the bottom line, which appears when I press right mouse button, remained the same.
Then I saved it to PNG file and compared to original colour.
Saved noise.png, Noise pattern is changed
Can I make this noise gone?