First, check for a bent blade.
To do this tilt mower (gas side up, oil side down) on one side and observe blade. Try to rotate it to see it it's hitting anything.
If it's not the blade, and it's vibrating severely and/or making banging noise, it's probably a bent crankshaft. You'll have to have it replaced, or buy a new mower.
getterdun, July 2010