Hi Karla -
Noises are sometimes a bit difficult to diagnose without being able to hear it... But my first thought is that the engine might be what*s called *dieseling* or wanting to keep running after you shut it off for a few seconds to a few minutes. This happens when there is too much carbon build up or there is unburnt fuel which allows the engine to keep trying to run after you have shut it off. There are other possibilities, it may be that what you are hearing is your exhaust cooling down which is normal. Another possibility is a cooling fan bearing that is starting to get noisy.
If it sounds to you like the first one I described - like the engine is still trying to run for a few seconds before it shuts off completely - than I would suggest getting it checked out. It may be as simple as a tune up, or there may be something larger going on. Either way, the longer you let it go like that the greater the chance is that it will cause additional damage.
Remember, when you bring it in, the more information you can give the technician the better. It will save her time and increase the odds of her diagnosing it properly and likely save you money in the process. Things like; has your check engine light come on at all, what kind of fuel you have been using, any special conditions the first time you noticed it, do certain conditions (like a hot or a cold engine) make the noise worse or better?
〉 Answered on Feb 21st, 2009 by Bogi Lateiner, Owner and Technician at 180 Degrees Automotive