What causes that noise? I've always wondered
 

You know the one. The weird knocking/clunking that you usually hear in an old chevy pickup when its trying to be shut off but wont shut off?

Its also the noise used as a sound effect in movies and cartoons when a car breaks down.

what causes this? an extreme lean condition? the engine rattling back and forth in the engine bay? or what?

I'm not having this problem, just heard it again today and started wondering what mechanically is going on to cause it.

It's called dieseling, and is very common in carburetted engines. The main cause is having the idle set too high. What happens is when the engine is hot, and you shut it off, the throttle plate doesn't close far enough, and the rotating assembly still manages to draw enough fuel out of the carb to keep it kinda running. The actual heat in the cylinders is what ignites the mixture, rather than a spark plug, which is why it's called what it is.

Take care,
-Chris

I believe you usually have to have carbon deposits/buildup on the pistons and heads to accomplish this as well. The carbon buildup is still glowing after the engine shuts off, which ignites the fuel as PKRWUD stated.

Somehow I knew that you would be the first to reply Chris:)

Thats a good explanation on why it wont shut off, but where does the horrible noise come from? Just the engine rocking back and forth?

That special sound
 

That sound you hear are valves, crank, rods and pistons jerkily attempting to operate but failing. Valves rattle, pistons clunk...not a pretty sound, as you've noticed...and no good for the rotating mass, either. Sad.

I cant be certain, but i think if the timing is off, it could cause this, but i might be wrong, wont hurt to hit it with a light.

Vacum leak can also do this.