Brake noise - please advise

Vehicle is a '92 Jeep Cherokee:

I've recently been experiencing some strange noises while braking. I would equate the noise to a "crunching" sound (sort of like a tin can being crushed). It only occurs when the Jeep is going very, very slow (almost stopped) and is rather inconsistent in occurance. Usually when it does appear, it will crunch for less than a second and then go away. It doesn't seem to be having any effect on the braking ability.

Does anyone have an explanation for this? Thanks.

I found a good reproducer for the problem today at lunch. If I put the car into gear, either reverse or drive, and put my foot on the brake (enough so that the car won't move, but not as hard as I can push it in) while turning my steering wheel around and around... the crunching noise is very apparent.