WHY IS MY ENGINE VIBRATING?
The first thing that came to my mind was...... Something is wrong... LOL..... Does it do it all the time? during start-up? when was the last ime a tune-up was done, it could be plugs, wires, cap, rotor, coil. many parts? If you can be more detailed. That would be helpful on pin pointing the problem, as there is a ton of things that could cause a vibration.
you could have a cracked harmonic balancer or as casper said one of many.
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