Either something in down in the disposal that is slow to grind up (like a bone, fruit seed, spoon) or one of the grinder hammers is corroded and frozen in place.

Disconnect it. Then you can often remove the rubber "flapper" with the unit in place.

Then inspect the unit and remove any forgein objects and see if the hammers move freely.

Thanks for your ideas. I checked the disposal carefully. The disposal is completely clean--no foreign objects. The two hammers move. However, one of them doesn't move as smoothly as the other one. Could this be enough to cause the loud noise and vibration? The hammers appear to be held in place by some sort of bolt. If the hammer is the problem, how would I go about removing the bolt and replacing the hammer? For that matter, where would I even find a new hammer for a disposal? Is this a do-it-yourself project, or should I just give up and call a plumber?

I know of one person that just breaking free a frozen hammer was enough to solve the noise problem.

You want to look under and around the unit to see that it is still well mounted and that there are no cracks or leaks in the GD.

If you don't see any problems. Then try running a couple of trays of ice cubes through it (along with water). They will help clean the unit.

If that does not help then probably time to replace it.

I did have one once that would throw the hammerr out of the unit. Litteraly. I did try to fix it and took it apart. But could not get the post out that held the hammer. There was a repair center in the city that I took it to. I think that they replaced the whole top plate and both hamers. But don't know. Deccided it was probably not workth it.

Get the model number and searial number and check on the warantee. I think that some of the top end Insinkerators go up to 10 years, in home. Others are limited to 1 year.

Replacing a unit are generally not too hard, at least not for the Insinkerator and Sears unit. The mounting ring stays in place and moving the "ears" counter CCW release it.

For the electric some using cordsets which can be moved from the old unit. Others are hardwired in place and are a little more difficult.

Great advice. Manually moved the hammers and it works like new.

I used a "curb key" (T-wrench for turning water off at the street and a wooden hammer handle to brace the can mechanism. Took me a couple of tries, but it worked to loosen the frozen hammer on an Insinkerator. If you stare at it long enough, you'll probably figure it out. :)

Just did the ice cube trick. Totally worked !

Just remodeled my entire kitchen and this was a problem right after completion. Called the contractor and told him he needed to come fix. Came across this and used the ice trick, worked awesome. Thanks.

The ice cube and lemon juice worked for me. Freed up a big chicken bone that had gotten wedged in there. Thanks for the suggestion, saved us $100.