Mine was making this same noise. It started on friday and was slowing getting worse... I was on a weekend trip and got home, got under my Xterra, checked it and I have oil all over the passenger side front suspension. I think either a shock blew, or a bearing went caputs. I'll have to take it in and find out. Mine has 12k on it.
Another thing, much worse happened to me this weekend. I went 4 wheeling on Friday and Saturday. SUnday I was driving along a dirt road to go mtn biking and guess what... BAM, like a shotgun... My F'ing side impact airbags went off. On a dirt road. It wasn't the smoothest dirt road, but I have had my Xterra pretty sidewise and taken it on some pretty rough terrian and they didn't go off.
As you can imagine with the airbags and bearing or shock that blew, I'm pretty pissed right now. Plus I don't know if I can trust my Xterra now.. the reason I got it was so I could have some fun 4 wheeling and have a nice daily driver too.