DIAGNOSIS The shroud around the flywheel/fan has come loose or cracked. The mounting tabs are plastic and can break, letting the shroud vibrate against the fan blades.
FIX Shut the generator down and let it cool. Remove the main cover. Inspect the large black plastic shroud covering the flywheel. Look for cracked mounting tabs. If tabs are intact, ensure all screws are tight. If cracked, the shroud needs replacement.
IF THAT DIDN'T WORK A foreign object like a twig or piece of debris could be stuck between the shroud and the cooling fan. A visual inspection with a flashlight will reveal it.
HEADS UP Do not run the generator with this noise. A broken shroud can dislodge and cause catastrophic damage to the fan and alternator windings.