Why is there a loud crack crack crack like a stone hitting metal from the back of the car near the exhaust when the engine is turned off and cooling down?

Your hot exhaust is cooling down and the metal is making a ping sound, chances are though, that your exhaust is possibly getting too hot, or has some loose components inside or bolted to it.

If your exhaust rattles at all, you may need to replace the muffler, and if you car has a trouble code related to misfire, or catalytic converter there may be too much fuel in the exhaust and is too hot.