Worst case scenario: you've dropped a tooth off the pinion or crown wheel. Happened to me in a Mazda 323 4WD turbo. There was a clicking sound that was Dependant on car speed, not engine rpm, so definately drive line related. Thought it was a CV joint. Replaced a drive shaft - noise still there. Was reasonably annoyed (to put it mildly) so dumped the clutch and ...... crunch, crunch, crunch..... stripped more teeth of both the crown (groups of teeth in about 4 places) and pinion (only had half the teeth left). Do what Beandip says. Also turn the car off, stick a brick under one wheel to stop it from turning, and get someone to put a bit of resistance on the other wheel or put the e brake on slightly (not too much, you still want it to turn), and (with car out of gear) turn the drive shaft , to rotate the wheel, and feel / listen if the shaft or wheel appear to skip. It's might not be that serious, but if you have the back end up, may as well check for it. Another car I had was making all sorts of weird noise from a worn out uni joint in the drive shaft. Hope its not as serious as a broken diff.