Diagnosis
The gears in the oscillation mechanism are either stripped, misaligned, or have a broken tooth. The noise is the sound of plastic gears skipping or grinding against each other. It often starts intermittently and gets worse.
How to Fix It
Turn off oscillation and unplug the fan. Lay it down horizontally. You need to access the gearbox assembly, which is housed in the center column behind the grill. Removing the front grill (usually clips) will expose it. Inspect the white plastic gears. If one is visibly cracked or has missing teeth, that's your culprit. You can sometimes find the gear assembly as a spare part online by searching the model number.
If That Didn't Work
If the gears look fine, the issue might be the motor shaft that drives them has come loose or worn down. That's a much tougher fix, as it involves the main blower motor. Your best bet then is to just disable oscillation permanently and use the fan in a fixed position.
Heads Up
Don't try to lubricate these plastic gears with oil or WD-40. It attracts dust and makes a gummy mess that ruins them faster. If you must use something, a tiny amount of dry silicone lubricant on the shaft is okay, but keep it off the gear teeth.