if it's run-on or dieseling can be caused by a few things, bad gas is one, high comprssion due to carbon build up is another. this can really only happen in old carburated cars.

is this a turbo car? it may have a 'cool down cycle' for the turbo that keeps the engine running for a few minutes to keep from burning out the seals... generally that is only an aftermarket add-on. you may want to ask a dealer.

or if you shut it down right after driving it hard, the turbo may continue to spin for a bit causing noise (that's actually bad, if you're driving a turbo car hard, let it idle for a minute so the turbo can spool down and the seals don't blow).