Loud 'clunk' when engine was shut off?

[westwind77]

This past weekend I pulled into a parking space, put it in park, then about 20 seconds later turned the key off - then heard a loud 'clunk/thunk' from the engine bay! I waited a minute then restarted it, no issue, turned it off, no issue. I have been driving it all week and it has not happened again. Has anyone experienced this before? Any thoughts at what it could have been?
80k on it right now, oil level is perfect. Thanks!