DIAGNOSIS That's not normal engine noise. It's almost always something loose vibrating against the metal cooling shroud (blower housing) or the fan itself hitting something. Common culprits are the metal baffle inside the shroud, the shroud mounting bolts, or the recoil starter assembly coming slightly loose.
FIX With the engine OFF and cool, remove the spark plug wire for safety. Take off the top metal shroud (usually held by a bunch of 10mm bolts). Inside, check the metal heat deflector/baffle plates—they're often spot-welded and can break loose. Also, check that all bolts holding the shroud and the recoil starter are tight. Look for any debris like a loose nut or washer bouncing around.
IF THAT DIDN'T WORK Inspect the cooling fan on the flywheel for cracks. Also, check the engine mounts. If a mount is broken, the engine can tilt and cause the flywheel to lightly contact the shroud at certain vibrations.
HEADS UP Don't run it for long with this noise. A loose piece can break off and get flung into the spinning fan, causing serious damage. It's worth the 20 minutes to pop the cover and look.