There will be some sideplay (the rotor and turbine are retained on the nested input shafts,) but not much - just enough to rattle when loose. It should not be loose when mounted. This is common to most torque converters - there are three fluid elements (stator, rotor, and turbine) with only one fixed to the case (the stator.)
However, when the TC is mounted, there should only be a little backlash between the splines on the nested input shafts and the rotor and turbine - that's it.
I usually advise checking flexplate noises with a longish screwdriver and a small, bright flashlight. You gently prise the flexplate away from the block while you're looking up there (in about six places around the circumference) and look for the cracks - it's more reliable to check that way.