Similar problem, I'd like to know what the answer/solution is. What do the curators use to determine noise/grain and can we do the same to avoid rejection?
I have seen this, didn't feel it gave me any more information than the rejection emails do.
Do you have any recommended work arounds?
Be sure to shoot your content at the lowest ISO possible and be sure to view your images at 100% before submitting. The moderation team reviews your files at full size so they see a lot more noise than you would at a normal view.
Be sure you aren't adding grain or texture in post processing and keep in mind the "artifacts" rejection reason isn't specific to noise only. That's the most common reason for this rejection however images with sensor dust spots, chromatic abberation or excessive post processing filters or effects can result in this error as well.