DIAGNOSIS: A foreign object has entered the fan housing or the blower wheel itself is unbalanced or has a broken blade.
FIX: Unplug the unit. Remove the filter (see above). Shine a flashlight into the cavity and use needle-nose pliers to carefully extract any visible debris like leaves, plastic, or coins. Rotate the fan blade by hand to feel for obstructions.
IF THAT DIDN'T WORK: If no debris is found, the noise is internal. The most likely culprit is the blower wheel shaft bearings have worn out or the wheel has cracked. This requires disassembly to confirm and replace the wheel assembly.
HEADS UP: A persistent grinding noise will quickly destroy the fan motor. Stop using it until the cause is found to avoid a much more expensive motor replacement.