hi james any idea why my auto gearbox would make a clunking noise when i put it into R or D kind regards

need help on auto gearbox makes a clunking noise when put into gear what could this be

i have a 94 300tdi discovery auto the belts are making a noise on the belts only when i drive it took it to a garage he put wd40 on it and said i wont here it again 10 yards down the road it cane back he just as well have put ub40 on it lol also when the jeep is warm when i put it in reverse it...