Diagnosis
The plastic fender mounts loosen from vibration. The fender then vibrates against the tire or the mounting bracket.
How to Fix It
Tighten the two screws that secure the fender to the metal bracket under the deck. Do not overtighten and crack the plastic. Add a small rubber washer between the fender and the bracket if the screws are already tight.
If That Didn't Work
The fender itself may be cracked near the mounting points. You will need to replace the rear fender. It is a specific part for the G4.
Heads Up
A loose fender can eventually catch in the rear tire, causing a sudden stop or damage. Fix it immediately.