The noise you are describing does sound way too loud. Most only make a "whirring" sound unless they are grinding something they are not supposed to. You may have a defective disposal, or there might be a loose screw or something that fell inside the unit during installation. There may even be some kind of shipping material that should have been removed before installing it. There is probably a customer service number in the directions or printed on the box. Call them, you can have them listen to the noise over the phone and tell you if it sounds normal. I have a feeling they won't think it does.
I hope this helps