Toyota Repair: toyota 89, combustion chambers, contaminated fuel

Question
my carolla is currently sputtering when i turn my key in the off position, the sputtering lasts for about 30 seconds. Also it emits a loud shriking sound that comes from my belt area, i have all new belts, can you help me out please?

Answer
It sounds like the engine is "dieseling" this means that the engine wants to keep running even though the ignition is shut off, this can happen because of low octane fuel, contaminated fuel, the timing is too far advanced, there is a lot of carbon in the combustion chambers which raise the compression to the point where the fuel will ignite by just compression alone because of the extra heat created, try using a higher octane fuel to see if it helps or put in an octane booster into the fuel, check the belts to make sure they are tight enough, the belts may be new but if they are too loose they will cause noise.