Single "CLICK" noise when Homing/Printing

[cliffyk]

The click you describe sounds like (no pun intended) there is insufficient clearance between the platen and the FEP/LCD, so when the platen descends the Z-axis worm is pressing it into the LCD causing the stepper motor to "cog" (attempt to turn but "jump").

What process are you using to align the platen? I describe in this post what has worked best for me.