If it's an automatic I bet the flex plate is broken. Does it idle OK but just make a crap load of noise? If so flex plate.... They can sound from a ping ping ping, to a knock, to a spray paint can being shaken.
When you hit the starter it will make a clack. Then when shutting it down it will make a clack-A-T-clack a couple of times from the compression of the cylinders being forced over TDC. and the converter wanting to keep moving from innershia.
At times you can think it is a rod knock........