DIAGNOSIS Something is interfering with the fan blades. The most common culprits are a loose pre-filter, debris that fell into the top grille, or a filter that wasn't seated perfectly and is vibrating.
FIX Power off and unplug. Remove the outer shell. Check that the pre-filter (the nylon sleeve on the True HEPA filter) is snug and hasn't slipped down. Remove the entire filter assembly, look inside the empty unit for any foreign objects, then firmly re-seat the filter so it clicks into place evenly.
IF THAT DIDN'T WORK The noise source could be internal. Gently try to spin the central fan blade with your finger (unit unplugged!) to feel for roughness or obstruction. A persistent grinding noise might indicate a failing fan motor bearing.
HEADS UP Never run the purifier without a filter installed. The unbalanced airflow can strain the fan motor and will not clean your air.