I'm guessing it's a rear CV joint. (they're covered by the rubber boots) They are a type of U-joint that spin & flex and when the load shifts on & off them (when worn) they'll make clicking noises. I'd have a qualified shop or the dealership take a look at them. They are not really cheap or easy to replace but don't let it go too long, if they break they'll coat you ALOT more $$$ and headaches