So I recently just changed out a 4 by 8 piece of plywood in my upstairs master bedroom's subfloor because it was old and had a large dip/crack in it. I noticed downstairs that there is a weird ripple in my ceiling downstairs (which was recently painted) near the area where I pulled out that plywood. I hadn't noticed it before removing the plywood. Is there anything to be worried about structurally? Is it just settling? How can i remove this ripple and make the ceiling smooth?