NP205 - Moving Noise

[NVBUYER]

Hey guys and gals,

I have a question that you might be able to help me with. When I'm moving and my speed exceeds ~15mph I get a weird hum/metallic sounds coming from the transmission/transfer case area. It sounds like a grinding noise (possibly a bearing?) from the outside, but a buzzing noise when I'm in the cab. Here's the kicker- when I put her in 4HI it goes away. Also when I have been driving it for an hour or so it will go away.

For further detail:
-72 k1500
-350 330hp/380ft lbs
-Level II 700R4 from Bowtie Overdrives with an AA adapter
-NP205 that I had 'inspected' by my local AAMCO before the install
-3:08/3:07 combo
-New driveshaft + joints

The transmission and engine have only 3000 miles on them. Has anyone heard this similar noise? Any ideas where to start poking around? Should I spare myself the trouble and overhaul the transfer case? I'm thinking it has 100,000 miles on it. Thank you for any input you can provide!