Why does an engine knock just before is shuts down?

[RoscoePColtrain]

Why does an engine make knocking sounds just before is shuts down?  What is that sound?  Is it harmful to the engine?
The gas tank is not empty, does it have to do with the carburator?

[kab69440]

Does it knock and shut down, all by itself?  Sounds to me like it's overheating, and yes that is extremely harmful to an engine.

[RoscoePColtrain]

The car makes a knocking sound then shuts down, it doesn't apprear to be overheating though.

[John_Kunkel]

The knocking is called "after running" or "dieseling", it is running for a second or two after the ignition is shut off.

It can be caused by poor gas, improper timing or most likely too high idle speed.

[deputycrawford]

     I agree with John. I believe its dieseling. Check Idle speed and timing. If that doesn't work then do a static compression check.  Has teh car been rebuilt lately? When did this start?

[max]

The knocking is called "after running" or "dieseling", it is running for a second or two after the ignition is shut off.

It can be caused by poor gas, improper timing or most likely too high idle speed.

i have seen a vacuum leak do it as well

[RoscoePColtrain]

Thanks for the ideas, I'm gonna try them out.