Did you happen to get an oil change recently? I would try taking the skid plate off for a day or two, just to see if the noise stops. If it stops, lube the parts that touch and tighten it good. I hope that is your problem. Please post the results. Mine doesn't make that noise, but it seems that all of the problems you guys write about, I eventually get.

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Chris
'00 Xterra SE 4X4 Auto(Silver Ice)