In the pic, your front motor mount appears to show open space in each side of the down tube. That open space on the down tube needs to be filled with something dense.
-You could possibly cut some pieces of wood to fill that space. You could make it from wood scraps from a construction site.
-Canadians will sometimes cut pieces of a hockey puck to fill the open space inside the motor mount. Use what you have on hand to fill the front motor mount. I don't think simply tightening the nuts will fix it.