The rattling and the difference of the resistance between the left and the right side of the flap come from the allowance given to the internal structure of the flap. Hinges located at both sides of the flap have some room in their internal structure so that the flap can be closed easily. Sometimes when you start using the computer, you may feel a kind of resistance to close the flap because the parts of hinges do not move smoothly. When both hinges move with different smoothness, you may feel that only one side of the flap rattles.
This loose feeling is normal for the computer. Please use the computer as it is, since there is no defect on the computer.