I have a 2000 Frontier V-6 4WD automatic. The last few times I've used 4WD (in 4H setting), I get a sporadic knocking noise under the floorboard - feels like it's directly under the accelerator pedal. You can hear it AND feel it - it goes on randomly for a few miles, then seems to settle down and go away. I first noticed it when coming out of 4WD and back to 2WD a week or so ago. As I said, after a few miles, it quieted down. Today, in a snowstorm, I put it into 4WD and got the same thing. Once again, it quieted down after a few miles. The noise sounds like metal hitting metal - it isn't constant - sort of random and sporadic.
Anybody have a clue what that's all about?