NP 231 Transfer Case Noise

[bmoore]

I have a 1988 Jeep Wrangler with a NP 231 T Case when its in 2wd it makes no noise, but in 2wd and hubs locked it makes a grinding/rattling noise when going down hill with gear pressure holding it back, also makes same noise in 4wd in the same situation, but makes no noise when in a pull, Please give me some ideas what you thinks wrong and what can be done to fix it.