engine noise?

hey, when i turn my engine off, every once in a while there is a last minute clack sound, just one! clack. doesn't do it all the time.
what do you guys think that is. as soon as the engine is swiched off before the last second it sounds like you hit a pan with a stick?
got any ideals

It's normal. What you're hearing is the sound of hot metal cooling down. When it cools down, it starts to contract.