I broken flexplate will sound like rodnock at idle. But once you give it light throttle or hold the rpm's over 1000-1500rpm it will go away. Rodnock will obviously get louder with more load/rpm. If you do have rodnock there will most likely be metal shavings or gold flakes in the oil when you drain it. You can also inspect the flexplate by getting under the car and taking off the inspection cover. Look for damage while somone turns the motor over via the crank bolt.