I did a bathroom a couple of years ago in my cottage. Screwed 3/8 Durock over Quickset and a solid 3/4 "tongue and groove plywood subfloor. Filled seams per Durock instructions. Finally, layed 2" tile ( in sheets ) on Quickset.
Each winter since installation I can hear 'cracking' under the tile on the central portion of the floor when I walk on it. This year I noticed that the middel section of the floor is raised up about a 1/8 inch to the extent that the floor flexes when I push on it. Strangely, during the summer, no cracking and the floor is smooth.
My suspicion is that the reduced humidity during the winter is contracting the subfloor and the Durock has buckled.
Now it looks like I'll have to tear out and start all over again. What did I do wrong??