Your Mustang's spring isolators main purpose is to reduce noise associated when the supension goes through its normal range of motion. If the spring isolators are missing, you would have metal to metal contact which would produce unwanted creaking and popping in the rear suspension during load shifting. The Mustang spring isolator is located between the spring and the control arm. We recommend replacing your Mustang's spring isolators when installing spring kits and/or for restorations. Helps to dampen road noise and vibrations!