It is usually going to be some loose rivets. Put a little water in it and see if you can see it leak out while on the trailer. Water very heavy so do not go over board.
If you have access to the back side (rivet tail), make friends with an A&P mechanic. Airframe-Powerplant. Rivet Gun, Bucking Bar. Bullfrog looks like a version of the cherry-max type gun, another type of blind (pop) rivet.