because the engine is still processing the oil for about 20 seconds after you turn the car off
The frist thing I would check would be the engine mounts.
No not necasairly
Noise coming from engine in Oldsmobile Cutlass could be as a result of an oil leak in the rear of your engine. This makes the gears to wear off leading to severe knocking noise.
It is called pre-ignition, find a friendly mechanic.
GASOLINE ENGINE OR DIESEL?
check engine mounts
The engine is cooling down. (or at least that's one reason for it)
There are some unknowns here. 1. Did you shut it off and it made a noise, OR 2. Did the car stop running suddenly, making a noise? If you shut the car off, some quiet noises are normal. You may hear the fuel pump from the rear of the car for a couple seconds, and you may hear a solenoid under the hood for a second or two, or the electric radiator fan after the car is shut off. The other question is what kind of noise was it? A loud bang? A whoosh noise, a scrape noise? Did the engine seem to try and keep running? Please add more detail and I'll try to help.
Where and when it makes the noise is imortant information. Most common is power steering pump when turning wheel quickly: Check power steering fluid level. If low, add proper fluid (check with engine off!!) Also, noise when driving, esp. over rough road going slow, ball joints: lube font end.
sounds like you need a valve job. how many miles on this car?
That would be the connecting rod bearings and the crankshaft bearings saying goodbye. You have succeeded in destroying the engine.
I wish i had the answer my car is doing the same. Hope engine is not blown..