I experienced very similar symptoms in my 2012 LEAF SL: A creaking noise when going over low-speed speed bumps (which looked like the city's standard "two foot residential" style) at low to moderate speeds. Like the OP, it seemed to me that cold weather (temperatures in the 30s of degrees F) aggravated the problem. The best way I could describe the noise was to say it sounded like two Styrofoam picnic ice-chests rubbing against one another.
The dealer's tech took the car out for a brief spin and confirmed my report of the noise, adding that he also heard a clicking sound from the CV joints (I had heard a clicking, but had put it down to having rocks in one of the tire treads). The dealer's shop (Wilsonville Nissan) told me they'd need to order a repair part, and had me come back the next day. They corrected both problems at no charge. I can't find any record of their remedy for the clicking joints (I believe they said that they just lubed them, same as other folks have reported here), but they did list what they did for the squeaks:
"VERIFIED CONCERN OF CREAK/SQUEAK NOISE OVER BUMPS AT LOWER SPEEDS. ISOLATED NOISE TO SWAY BAR BUSHINGS, REPLACED BOTH SWAY BAR BUSHINGS AND ROAD TESTED THE SAME AGAIN. VERIFIED NOISE NO LONGER HEARD ON ROAD TEST AFTER REPAIR"