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The humming indicates that the motor is still trying to turn, but the grinding mechanism is jammed. First things first! Turn off the power and look inside the grinding chamber of your disposal a flashlight to see if there is any visible object... large staple, coin, dentures... that might cause the disposal jam. If there is some nondescript object in the disposal (nobody would recognize you either if you just took a ride in a wild garbage disposal) you must remove it before testing. I use a special extraction tool... a simple magnet attached to a telescoping metal rod. It looks like the antenna from a cheap portable radio, actually! Unfortunately, the magnet will only work on steel and iron objects.
You may need to use some ingenuity to remove coins, which are generally not magnetic. Duct tape rolled up onto the end of a pencil might help you grab it, or even a piece of chewing gum! Two butter knives used like chopsticks can also be effective… use your creativity. Please… be sure the power is off before trying any of these improvisational techniques!
If you find a very small item, such as a staple or other non-metallic object that just won't go through and defies all attempts at removal, there is a trick I learned from my middle daughter.Load the disposal with ice cubes and turn it on while running cold water through it. The ice stops the small object from jumping away from the grinders so they can pulverize it. Again, this is only good for smallish objects, not for little stones or coins!
Sometimes you will see an object in the chamber but it will be impossibly jammed between the grinding mechanism and the side of the disposal. Or sometimes you will see nothing. In either case the only way to release the jam is to turn or "spin" the grinding mechanism by hand… the disposal's motor is not powerful enough to do the job itself. Virtually all the newer disposals have a hex head slot directly under the center of the disposal. To see it, use a mirror and a flashlight to look underneath the disposal. By inserting an appropriately sized Allen head (also called hex head) wrench into this hole, you can gently turn the disposal mechanism back and forth to free the blockage. Sometimes by "spinning" the disposal mechanism with the wrench, the offending item passes through the disposal like a rock... other times it pops back into the chamber and taunts you! Check the chamber again for any loose material. Then and only then can you turn on the water and test the disposal. You may have to repeat this procedure a few times before you get a gold star!
Most reputable installers leave the correct turning tool with the disposal, though they tend to "disappear" with multiple homeowners and tenants. Search under the sink first. It might even be in a plastic pouch glued to the side of the disposal! If not, you will have to go to a hardware store and buy one.
If you are unfortunate enough to have a disposal without a hex head slot underneath, you will have to spin the disposal with the use of a piece of wood and creativity. Insert the piece of wood… a 1"x2" board or even a broom handle will do nicely… into the chamber and deftly push it against the grinding arms to loosen the mechanism. Be patient and try to work in both directions until the mechanism spins freely. Don't try to put a wrench on the exposed nut in the middle of the chamber… this holds the whole shebang together and loosening it may cause an unexpected disaster!
Worst case… the jam is total and cannot be freed with either method. In that case, the best solution in my opinion may be to replace the disposal. Yours is not really that old, but the labor costs to remove the existing unit, disassemble it and "possibly" fix it may not be worth the risk.
From your brief description it sounds as if one of two things may have happened. Either you dropped something into the disposal that jammed it OR the disposal has self-destructed. When a disposal jams, the grinding mechanism stops turning. For a few seconds, the motor may make a humming sound... followed by silence as the built-in circuit breaker turns the motor off to protect it from damage.
First, turn off the electricity to the disposal at your main electrical panel. Though disposals usually don't self-activate, if you want to be totally safe turn off the disposal circuit at the main panel! Look inside the disposal with a flashlight for anything hard... metallic or even a small stone... that might be hopping around.
If you do see something, remove it with a pair of pliers, long tweezers or even a magnet if it's metallic. Some folks use their hands. I don't recommend it... even with the POWER OFF you won't get "disposed of" BUT if your hand gets stuck we may next see your pretty face on "Rescue 911"!
If you see an object that is solidly jammed in place, you need to rotate the blades to attempt to free it... more on this below.
If you don't see anything inside the chamber, reset the disposal and try operating it again. There should be a reset button on the side or bottom of the unit (under the sink). When you press it, you will feel and probably hear a "click" indicating that the built-in circuit breaker was tripped and is now reset.
Then, turn on the power to the disposal and try to operate it while running water through it. ALWAYS run water through your disposal when operating it. If it either makes an unusual banging noise of just hums, turn it off immediately. Sometimes, small items such as staples will drop through the disposal. Sometimes, larger staples or other small metal objects will jam between the cutting blades and the sides of the disposal chamber. This would cause the "humming" as the disposal motor wants to turn but can't.
If this is the case you need to rotate the blades to free them up. This is accomplished by either using a hex-head wrench underneath the disposal (the tool comes with all modern disposals and may be under your sink) or inserting a broomstick or other solid wood stick into the top of the disposal and attempting to turn the blades in either direction. Some disposals have a retaining nut in the center of the blades which can be used to rotate them IF you have the proper size socket and a long enough socket extension. Turn clockwise only, though... you don't want to loosen this nut.
Once the blades are rotating freely, turn on the disposal and listen... most often the offending material has dropped through the disposal into the drain never to be seen again. Sometime the turning will just loosen the jam and you will have to look inside again to remove the object.
If you cannot rotate the blades at all, you may need to have a pro disassemble the unit to free it up. Or, if the disposal is over 10 years old, my advice is to seriously consider investing in a new one!