Noise in driveline in 2002 Z71 Tahoe

[Fortune50]

My buddy's '02 Tahoe Z71 4wd makes a noise whenever the 4wd is engaged. It has the auto 4wd feature. It only makes the noise in 4wd hi. It's not terribly loud but it's there...do you think any noise could be normal or should it be undetectable. Just would like some opinions or advice. All the fluids were flushed and the front differential was quite dirty to say the least but everything else looked good in the rear differential and the transfer case.

[Fortune50]

Some more questions about the Auto 4x4 feature on that '02 Tahoe Z71.
In 2 Hi mode are the front differential gears and front driveshaft still turning? OR are there lockout hubs that disengage the front differential from the front wheels? In other words, in 2 Hi mode the transfer case is only powering the rear driveshaft, the front hubs have disconnected the front wheels from the front differential, so in turn the front differential and front driveshaft are NOT freewheeling so to speak. Or are there NO locking/unlocking front hubs?

...so here's my take on each mode.

2 HI mode: transfer case is only powering rear driveshaft and if the front hubs are the locking/unlocking type-they have disengaged the front wheels from the front differential which allows the front differential gears and front driveshaft coming out of the transfer case to stop turning altogether even when the vehicle is moving.

4 Hi mode: Transfer case has connected to both front and rear differential driveshafts, AND if they are locking/unlocking front hubs, they have now engaged the front wheels to the front differential.

Auto mode: transfer case has disengaged from the front driveshaft, but is engaged to the rear driveshaft, and if they are the unlocking/locking type, the front hubs in Auto mode have locked the front wheels to the front differential. If slip is detected, then the transfer case engages itself to the front driveshaft which in turn powers the front wheels. Or am I wrong in this description?

As you can see I'm trying to figure out what ALL is going on in each of the modes so I can better track down the problem with his Tahoe making that noise ONLY in 4 HI...
So are there locking/unlocking hubs in his '02 Tahoe Z71 4x4 with the Auto 4x4 mode feature?
And can anyone tell me EXACTLY what's going on in the driveline componants in each mode. Thanks again.