I purchased engineered wood flooring at my local flooring store and they subcontracted the installation. My bedroom's concrete (slab) floor is about 2 inches lower than the marble tile in the hallway that leads to my bedroom. In order to raise the bedroom floor to make less sudden transition, the contractor decided to lay fiberock tile backer board on the floor on top of a high-end type of thinset-type of material (I forgot the name of it) that the manufacturer says can be laid up to 3/4 inch thick. So the heavy duty thinset-type of material added 3/4 inch, the fiberock added 1/2 inch, and the engineered wood, after it is laid down will add 1/2 inch, bringing the height difference between the 2 rooms to only 1/4 inch. So they laid down the fiberock a couple of weeks ago (there was a delay on the wood installation) and they used a wedge/spacer leveling system to minimize any lippage. Now, when I walk on the fiberock, it sometimes randomly makes a very loud popping sound, almost like bubble wrap or popcorn popping. Not a bunch of pops at once but just one loud pop. And I usually can't replicate it if I step in the same spot twice. But hours later I can sometimes make it pop again in the same spot. The flooring store and the contractor can't figure out the cause of the noise. They screwed down some of the fiberock using tapcon screws and the floor still pops randomly when walking on the boards. At this point, they are running out of ideas and are thinking that the spacers from the leveling system are somehow rubbing in between the fiberock boards and the slight movement from walking on the fiberock pushes the plastic spacer and is making the popping sound. I think it's more likely that it's the thinset/mud material under the boards making the sound but they are saying that would be more of a crunchy gravel type of sound, not a single pop. Any ideas what can cause this kind of noise? We obviously don't want to glue down the engineered wood until we eliminate the popping sound. Thanks!