Constant velocity joints are fairly tough by nature, but even the best of them fail over time. CV joint-failure symptoms are fairly universal, and often progress rapidly from a mild clacking to total failure.
Clacking and Thumping
Clacking and thumping are universal symptoms of CV joint failure: a slight tap on acceleration and deceleration that progresses to a hard thump. That slight initial tap is the sound of the CV joint's internals moving around and smacking against each other when you hit the gas or brake. That tap is like a slight hammer blow, and every time it hits, the clearances get a little bigger. Ultimately, you end up with a hard, metallic thunk as the CV joint hammers itself to bits.
Vibration is the final stage in CV joint failure and generally is a result of complete breakage inside the CV joint. Once the CV joint's internals break, the rubber dust boot connecting the CV joint base to the shaft is the only thing holding together the assembly. Any imbalance in the axle shaft as a result of the breakage causes it to flop around inside the dust boot and throw the assembly off balance. The result is very similar to that of an unbalanced wheel: a vibration that worsens with speed.
Total Failure Possibility Number One
You're looking at two basic scenarios if the CV joint fails completely. The average "open" differential transfers power through the path of least resistance, meaning it'll always go to the shaft that spins the easiest. Once the dust boot loses its grip on the shaft or rips in half, power defaults to that CV joint and it spins without moving the car. The result: You're on the side of the road and going nowhere.
Total Failure Possibility Number Two
Slightly less likely is the idea of "wrap-up." Wrap-up happens when the CV joint completely disintegrates but the dust boot doesn't lose its grip on the shaft or tear in two. When the differential sends power to the CV joint axle stub, it twists the rubber dust boot around on itself. This twisting shortens the boot, thus pulling the axle shaft out of the transmission. While the axle shaft may not fall completely out of the transmission, a half-engaged shaft can wreak all sorts of havoc on the differential assembly.
Inner vs. Outer CV Joints
CV joint malfunctions are fairly simple to track down. Your axle shaft has two CV joints: one near the transmission and another behind the wheel. The outer CV joint does most of the work, since it must cope not only with up-and-down movement, but side-to-side movement when the wheel turns. An outer CV joint tends to pop and bind far more while turning than an inner joint. The acid test is to crawl under the car, wrap your hand around the axle shaft and try to rotate it and move it back and forth. If it rotates with an audible click or moves back and forth at all, then you've got excess bearing clearance and need to replace the joint.