If its only 1x clunck when moving forward or reverse, this should only be BACKLASH, as DaveW suggests its distance allowance between all the little gears inside the rear diff (Planet Gears)each gear wheel requires a minute space, so you do not get lock up or seizure, the distance between each gear mulitplies (your Clunk)
Although possible to improve not always fully curable, IMHO best left alone and lived with, you will probably find it will go on forever.
This is a common noise exagerated by the fact you sit close to the rear of the car and it has very little sound proofing.
