Is it a single knock or periodic while turning? A bad CV joint will clunk in time with the rotation of the halfshaft. A single knock is most likely worn sway bar links, but could be any number of suspension components. I've just found sway bar links to wear faster on every car I've owned.
A clunk that's not coming from the front could be a loose spare tire or rear suspension components like shocks, shock mounts, or control arms.