When foreign objects, including coins, fall in the garbage disposal, they will make a loud clanging noise and may jam the disposal entirely. Most modern disposals come with an automatic safety shut-off mechanism, so if a penny jams the disposal enough, the disposal will stop working altogether. Removing a foreign object from your garbage disposal is a simple enough task, though you may have to get under the sink to do the job properly.
Unplug the garbage disposal.
Shine a flashlight beam into the garbage disposal while moving the splash guard rubber to the side. If you can see the penny, reach into the disposal with a pair of needle-nose pliers and pull it out.
Insert an Allen wrench into the hex-shaped hole on the bottom of the disposal unit.
Turn the Allen wrench gently to the left until you meet a obstruction and it won't turn any farther. Turn the Allen wrench to the right until you meet the obstruction. Continue turning the wrench, which turns the motor shaft, back and forth until the penny comes loose and you can turn the wrench all the way around.
Reach into the top of the disposal with the needle-nose pliers and pull out the penny.