Strange S60 loud clicking at slow speed

Criscobath said
I haven't been hard on the rear-end at all (phrasing). Maybe 10 full throttle blasts and 400 total miles.

the break in oil was prob a conventional and heavier. the synthetic is prob thinner and has made the noise audible. when i first drove my car with the locker it freaked me out but i dont even notice it any more and i have thousands of miles on it.