Brake or Transmission problem?

[skrizan]

I have 2005 Envoy 4WD with V6 and 50,000 miles. The problem that I have is not happening all the time which will make it hard to diagnose.
Starting this fall I noticed a loud drone when slowing down. (Not necessarily when applying the brake either, it makes it when coasting also.) It kind of sounds like when you downshift a manual transmission at too high of speed.
I had been using 4WD a little then so I thought it may have been stuck in 4 high. It also has AWD also.
Lately it seems to be happening more often, and the last time I was turning into a parking lot, let off the brake and still felt resistance, then I heard a clunk from the rear and the noise quit and the resistance was gone.
It was in AWD at the time. I have been switching it into 2High and back when it occurs, and it seems to go away, but I have no idea if doing that did anything, or it quit because I stopped and started again.
I didn't think AWD really did anything unless you lose traction.
I usually keep it in 2High unless the roads are wet or icy though.

The rear brakes look o.k., no extra brake dust or anything. Anything else I can check? I've done some searching but haven't been able to find anything that sounds like my problem.

[maxwedge]

Welcome to AF, this could be the transfer case is not disengaging and the suspension/wheels bind in sharp turns like it was in 4wd, this can be an issue with the controller in the transfer case which may need replacing or reprogramming or both.