First check to see that the spare tire is secured with the bolt-down attached, and that the jack is secure.
Under the car is a few possibilities, check exhaust, it's hung on rubber, move up and down and see if it hits. The rear sway bar link bushings are grommet type rubber parts in the lower arm. When they get crushed, clunks occur over bumps, especially at slow speeds. They can be replaced at reasonable cost. Good Luck
Al
