DIAGNOSIS Something is loose inside the unit or obstructing the fan. The most common culprit is the pre-filter not being snapped securely into its frame, causing it to vibrate against the plastic.
FIX Power off, unplug, and open the rear cover. Check the pre-filter (the black foam layer). Remove it, ensure it's clean and dry, and then re-seat it firmly, pressing all edges into the grooves so it lies completely flat. Close the cover until it clicks shut.
IF THAT DIDN'T WORK Inspect the main HEPA filter. If it was dropped or mishandled, the internal paper pleats can become deformed and touch the fan cage. Gently rotate the filter to see if the noise changes; replace the filter if it's damaged.
HEADS UP Never run the unit without all filters installed. The missing mass changes the fan balance and can cause significant vibration and noise, potentially damaging the motor.