What causes that noise? I've always wondered

[PKRWUD]

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