From what I saw inside mine, it is undoubtedly the battery flopping back and forth. It's pretty snug side to side. Mine is securely taped down to the aluminum underplate.
Look on the bright side: When you decide you need to open it up and replace the battery, at least it will be easier to take out
Go gentle on that twisting - the Kobo is pretty sturdy, but Gen3s and eSlicks tended to break their screens doing that - pretty much the same design, except for the case.