The flex plate replaces the flywheel in automatic transmission cars. It bolts to the end of the crankshaft and the transmissions torque converter bolts to it. Usual failure mode is the crankcase bolts or the torque converter bolts work loose, allowing the parts to start moving in different directions, the clicking sound you heard. Failure is a matter of luck and is beyond driver control… At 155K miles, I would have had the transmission rebuilt while it was out…