Clacking noise as right crank goes 'over the top' once per revolution
It's always difficult to diagnose noise descriptions, especially in the drivetrain.  But guessing by the:


  ... the bottom bracket moves without any resistance ... more noticeable on the lower gears ...


I would say the rear hub's freehub is damaged.  Pop it off the hub body and inspect where the pawls are (mind the pawl springs don't ping out!), look for broken pawls and/or mashed ratchet teeth (the teeth which the pawls engage with).  Post some pictures of it if you are unsure.