Diagnosis
Debris (like a leaf or plastic tag) is hitting the fan blades, or the internal foam sound-dampening liner has come loose and is vibrating.
How to Fix It
Unplug the fan. Remove the rear grille (secured by a few screws at the back). Visually inspect the fan cylinder for any foreign objects stuck between the blades and the housing. Remove anything you find.
If That Didn't Work
The noise is likely the black foam liner inside the housing that has detached. Open it up and use double-sided tape to re-secure the foam to the plastic interior walls.
Heads Up
Never try to remove debris or adjust anything while the fan is plugged in, even if it's off. The motor connections are live.