Well, you have something loose, worn, bent, or broken! Most likely loose, and maybe fixable without taking it all apart. Remove the screw that's visible, that should allow you to slide the arm part way off (note - there's the motor wires inside, so you won't be able to remove it all the way, but enough to check the bearing etc.). Beyond that, there's an arm mount inside the drone that's attached to the body by 4 screws. That mount could be loose or broken, and would require disassembly to fix.
There's a rule in trouble-shooting, check what's easiest first, even if it's not the most likely culprit. In this case it means remove that little screw . . .