Diagnosis
Something is interfering with the fan blade (the vortex circulator). This is usually a piece of debris that got past the rear grill, or less commonly, a bent blade from an impact.
How to Fix It
1) Unplug the heater. 2) Look through the front and rear grills with a flashlight. You might see a lint ball, hair, or a small object. 3) Use a can of compressed air to blast through the grills from multiple angles to dislodge debris. 4) Gently try to rotate the fan blade by hand through the grill to feel for obstructions. If you see a specific object, long, thin tweezers might help.
If That Didn't Work
The noise could be coming from the fan motor bearings starting to fail. Listen closely—if the grinding is more of a constant hum or rumble from the motor housing itself, that's the likely culprit.
Heads Up
Never insert anything rigid like a screwdriver into the grills while the unit is plugged in, even if it's off. You could seriously damage the blades or motor, or cause an injury if it suddenly powers on.