This happens in multiple of my files / completely different renders. V2.83.2, Cycles renderer.
There's a noise to any dark area that isn't sample dependent (looks exactly to-the-pixel same at 500 or 5000 samples) and appears regardless of using the denoiser. What's weird is that if you do use the denoiser it unlocks a pulldown in the render for "noisy image" (which, presumably, is the un-denoised render) and in this case is actually the cleanest render - so I assumed that it was the denoiser itself generating the noise accidentally, but it's also generated when the denoiser is off.
So something is breaking in between the processes of generating the base format image and the final result 'combined' image?
Is this a bug?