what would cause a dull boing noise after car engine is turned off. on 1999 Buick Park Avenue

occurs in 2 seconds intervals for about 5 minutes or so after the car has been driven maybe 20 minutes.So heat of the engine factors into this happening. shorter periods of driving does not cause the noise to occur. This happens after the car engine is turned off.

Asked by for the 1999 Buick Park Avenue
Most likely thermostat sticking then rapidly opening causing noise have cooling system checked!
2 more answers , 1 more comment
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 that the exhaust may be clogged and overheating and you may be hearing the metal 'pinging' as it cools down.
your engine is decompressing(oil draining into pan). louder in some vehicles. rec. treatment with oil stabilizer on next oil change. this will recondition seals, and cylinders.
Thank you for your response will give your rec a try.