I had a clunk coming from somewhere below the front seats. It would occur on acceleration and when I ran over large bumps in the road at speed. I took it to the dealer and they found that the tranny/transfer case wire harness connector was hitting the frame rail under the co-driver's seat. They rerouted the cable and the sound went away. When I drove home I put the Xterra in 4wd and the indicator did not come on. I looked under the vehicle and found the wiring harness disconneted. I drove it home and adjusted the wires myself so there was enough slack to do the wire connector up plus ensure there was no contact with anything. It has not made a sound since.