what would cause a dull boing noise after car engine is turned off.

Have you opened the hood and checked coolant level? BOTH when cold (before first start of the day , and after it's been warmed up as you describe? OOOOOOOOOOOOOOOORRRRRRRRRRRRRR.........another wild guess....but I think wetry is most likely correct.......is that the exhaust may be clogged and overheating and you may be hearing the metal 'pinging' as it cools down.