Diagnosis
A foreign object is likely lodged in the fan assembly or the loop amplifier (the ring on top). This is common with high-speed airflow.
How to Fix It
Power off and unplug the machine. Remove the loop amplifier by twisting it counter-clockwise and lifting it off. Visually inspect the main fan bladewheel for debris like hair, dust bunnies, or small objects. Use a flashlight to look inside the base. Remove any debris carefully.
If That Didn't Work
The rattling could be a loose component. Check that the filter is installed correctly and seated flush. A misaligned filter can cause turbulence and noise. Also ensure the loop amplifier is securely reattached by twisting clockwise until it clicks.
Heads Up
Never insert tools or fingers into the fan bladewheel while the unit is plugged in. The fan can spin freely and cause injury. Regular cleaning of the loop amplifier and intake grilles prevents this.