I just installed a new garbage disposal myself and I'm afraid I might have done something wrong. There is no leaking whatsoever, but the entire countertop often shakes for a few seconds when I turn the disposer on. I'm wondering if I may have put on a unit that is too powerful? The unit is a 1hp Waste King that I purchased based on good reviews in consumer reports without stopping to think if 1hp was too much. The counter is laminate. I'll post some pictures if anyone thinks it would be helpful to diagnose the problem.
I'd suggest removing the unit from the underside of the sink and the plumbing, then turn it on and feel for vibration. If it shakes or vibrates hard when it isn't mounted then I'd take it back. It should run reasonably smoothly and it certainly shouldn't shake the sink, plumbing or countertops.