knocking noise heard when i turn off my engine

[mikecor]

hello all i have a quick question to ask and see if anyone can answer it.
here it is why does my engine make a knocking noise when i turn it off after its been run for more then an hours drive time. this does not happen every time i turn it off only some times and it will sometimes knock only once and other times it will give me three knocks. what could be the cause of this noise.?

thanks to all who answer me with this problem.

 

[Original_Ranger84]

Does it shutter a bit when you get the knock? If it does it kinda sounds like dieseling to me. The build up of carbon in the engine will get extremely hot over long drives and may be hot enough to keep the engine turing a time or two after it shuts off...

 

[Quartermile]

That sounds right, but I know it as run-on. Usually happens only in carbed engines, but yeah the heat ignites a few cylinders after being "shut down". Sometimes putting a can of sea foam in on half a tank of gas and running around with a little more gusto on the gas pedal fixes it.

 

[Original_Ranger84]

I always heard dieseling for the reason its going even without spark.
It can happen on FI engines too. Since the 3.9L probably has the TBI injection it is different from multiport and probably has a little more fuel left after shut off.