Sounds like a flexplate for sure. Mine just came loose, it didn't crack. The jackwagon that assembled my drivetrain at the factory didn't put any blue loc-tite on the crank bolts and a couple were backing out, and 5 were hand tight! Flexplate is an easy fix, and a cheap one, but it's labor intense pulling the motor. Might as well throw in some goodies while you're there though.