It sounds as if something has fallen inside. It's easy to check. First turn the power to the disposer off at the breaker panel (lock or tape the panel door closed) or use the wall switch TAPE the switch in the off position. Get a flashlight and use a spoon or something to move the rubber splashguard out of the way and look inside. 9 times out of 10 a foreign object has got in there. You would be surprised at the noise a bobby pin will make

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WARNING</u> metal objects caught in a disposer can have VERY sharp edges. Use a pair leather gloves, or if you don't feel comfortable with putting your hands inside a pair of BBQ tongs are good for retreiving stray objects.
Good Luck