Sounds like fender to me. A couple things to check. Fender mounting, and axle nuts. If you can wiggle the fender on it's mounting bolts, you may have to remove the back wheel to get at the bolts. Then check chain tension to see if one of the nuts has loosened. Center the wheel between the chain stays. Then lastly you may have to have the spokes tensioned to true the rim. Checking and tightening fasters is regular maintenance on bikes.