Diagnosis
The front fender is loose or cracked. The mounting bolts are rattling or the fender is hitting the tire.
How to Fix It
Tighten the two fender bolts. They are accessed from underneath the fender, near the fork. Use the correct hex key. Do not overtighten into plastic.
If That Didn't Work
The fender itself has a hairline crack near the mounts. It's flexing and hitting the tire. You must replace the entire front fender.
Heads Up
A broken fender can suddenly catch the tire and lock the front wheel. This will throw you over the handlebars. Fix it immediately.