2000 Chevy Tahoe Tranfer case

I have a 4x4 2000 Tahoe with an automatic transmission. When I put it in D or R I get a brief loud metallic noise which appears to be coming from the transfer case. To confirm this I placed the transfercase in natural and placed the transmission is D and R and got nothing. I then placed both the transfer case and transmission in Nautrual. Got under the car and turned both drive shafts at the same time and got an identical noise. Does this sound like I need to replace the transfer case, rebuild or what? Or is my diagnose not even transfer case.