Check the front lug nuts as the Wolfman has suggested. If they are tight, the next thought is that the universal joints will sometimes make a clicking sound in reverse if the cups are worn. I would put the bucket down and lift the weight off of the front wheels and then spin the wheels in reverse and feel and listen for any roughness or sounds. If you find roughness, consider draining the front axle oil into a clean pan, and inspect the oil for metal particles. If you find metal particles in the oil then the front axle will need to be disassembled to see what parts have failed.