Here's a shot of my mount after I replaced it. Curiosity got the best of me and I cut it in half using my band saw.
The concentric rings of rubber at the top are what actually holds the engine up. That rubber pad in the center will sink and come in contact with the center part when the mount gets weak. This contact is what causes the vibration. Normally, there is damping fluid in the mount and it acts like a shock absorber.