I'm just posting this in case it can help anyone else who is doing an internet search for this problem.
My 2011 Xterra was making a clunking noise under the driver's side floorboard when going over smaller bumps or seams in the road. I could even feel it with my feet. An internet search brought up all sorts of possible solutions. Some big repairs, some small.
What mine turned out to be was the left front upper strut mount, and it just needed to be tightened up. It wasn't even broken. Noise is now gone.