If you let the air out of the tube and then pry back the tire's side/bead to view inside the rim along the bead seat you might see a bit of the tube poking between the tire and the rim, where the tire bulges. Often at the valve because of the valve's base being a thicker wall thickness then the rest of the tube. The fix would be wiggling/prying the tire back and forth until the tube fits within the tire completely. Andy.