Diagnosis
Mechanical interference in the oscillation gearbox. Debris or a worn plastic gear tooth is catching.
How to Fix It
Turn the fan off and unplug it. Manually rotate the fan head through its full range of motion by hand. Listen and feel for grinding. Inspect the rear grille and base for any visible obstruction like a cable or toy. Tilt the fan back and gently shake to dislodge any small debris inside the base. Plug in, turn on oscillation without fan speed. Listen.
If That Didn't Work
The nylon oscillation worm gear may have stripped teeth. This requires disassembling the base to inspect the white plastic gear assembly. A common failure point after years of use.
Heads Up
The oscillation mechanism is not designed to be lubricated. Applying oil or grease can attract more dust and damage the plastic gears.