Yes and yes. Rattle seems concentrated worse in center area of screw.
Yes, original half nut, locked down tight as possible.Are you using the lathe's original half-nuts? (Sorry, I have been following your build but my memory isn't what it used to be...)
Drill ran much slower. If I run the axis at half-speed, rattle is barely noticeable. Drill also ran the very same coupler--And it was DEFINITELY misaligned with drill.I have just viewed your early video of the Z axis driven by a battery drill. The rattle isn't apparent, so something has either come loose, or it has to do with the stepper motor end.
It almost seems like the screw is whipping, although I can't imagine that 3/4 inch screw flexing that much. Maybe it IS whipping in the looseness of the half-nut, but I can't see or feel it. The half nut feels really solid.
CR.