Diagnosis
The engine mounting bolts have probably worked loose from vibration, causing the entire engine to shake against the deck. This is a frequent issue.
How to Fix It
Tighten the four engine mounting bolts. Disconnect the spark plug. Locate the four bolts (usually 12mm heads) that secure the engine to the mower deck—one at each corner of the engine baseplate. Check each with a wrench and tighten firmly. Do not overtighten to the point of stripping.
If That Didn't Work
The vibration could be caused by a severely unbalanced or damaged blade. Remove the blade (mark its orientation first), check for bends, cracks, or heavy damage, and either sharpen/balance it properly or replace it.
Heads Up
Always use a blade removal tool or block of wood to keep the blade from spinning when loosening or tightening the blade bolt. The bolt has standard threads (lefty-loosey, righty-tighty).