That system is my specialty, as it is for many offroaders. Read this first for a detailed theory of operation:
Offroadtb.com Transfer Case Operation: NVG 226 (NP8)
Here's more info on the splined disconnect and how they can fail:
Offroadtb.com Front Axle 4WD Disconnect
In a nutshell, In 2WD, the splined disconnect is not connected, but the driver's side wheel spins its CV shaft and thus some internal parts in the front differential. All the time. So you have to keep up on front differential fluid changes, we recommend every 100K. How many miles do you have, and how's your differential fluid level? Also how's your rear differential while we're on the subject?
The passenger side tire always spins its CV shaft, but the splined disconnect does not couple it to the intermediate shaft that goes through the tube cast into the oil pan.
In A4WD, 4HI, and 4LO modes, the splined disconnect engages, and now the passenger side tire is spinning the intermediate shaft, and more parts move inside the front differential. A differential with low fluid or internal damage tends to make more noise in these modes than in 2HI. But it's possible for a bad differential to make noise even in 2HI. Some folks assume that nothing inside moves when you're in 2HI mode, but that's not true. Only if we had disconnectable HUBS would that be true, and they didn't design those in for us.
Your other major source of noise is the transfer case, with MANY moving parts, especially a clutch pack. The major difference between A4WD and 4HI mode is the position of the clutch pack. In 4HI, the clutch is engaged fully, and sending as much torque forward to the front driveshaft as it's sending backwards to the rear. In A4WD, the TCCM is looking at wheel speed sensors, and sending only about 5% torque forward under normal conditions. When it sees the rear wheels spinning faster than the front, the TCCM assumes there's a need for more torque to be sent to the front, it commands the transfer case encoder motor to move a lever on the transfer case that adjusts the pressure on the clutch pack, and the clutches engage and (supposedly) smoothly send more torque forward to eliminate the wheel spin.
The constant movement of the clutch pack, and constant partial engagement of slipping clutches results in early transfer case destruction for nervous owners who leave their vehicles in A4WD mode when it's not absolutely needed. Sometimes they die in 6 months to a year. Lesson: A4WD mode is NOT AWD. It's not a Subaru.
The other huge note is that the transfer case fluid change interval is a surprisingly short 50K.
I can't stress this enough - deferring the fluid change can cost you a transfer case. What's your situation on fluid changes here?