The hub cones are loose. Remove the wheel from the bike, grab the axle and move it side to side, back and forth. You'll feel the axle moving relative to the wheel. The cones need to be adjusted. Should only take 10 minutes to make the adjustment if you have the tools and know how to do it. But overhauling the hub with new bearings and grease might be recommended as well. Drop the wheel at your LBS to get this done right away. You damage the hub by riding it like this.