I am stacking a still scene I took with my Canon EOS 6D in Photoshop to reduce noise. I've opened up raw files and stacked them with a smart object/median filter. However I've noticed something odd at the final result: there is some black square-ish pattern all over the image. Have a look at this 100% crop from the final result:
While the effect is not very strong, it is easily noticeable. I do image stacks and this is the first time I've ever seen such effect. Why does it happen and how do I get rid of it?
UPDATE: Here is an "increased-contrast" version of the same image, just to make the grid more clear: