If it sounds like your motors knocking you can pray its just a cracked flywheel. Sometimes it will only make racket at idle, and when you got a load on the motor it quiets down. I've had several crack on my truck right around where the flywheel mounts to the crank. Kinda hard to see with the transmission all together. You can try grabbing ahold of the flywheel and see if you can "flex" it back and forth.