hello brett,
from the sound of your description it sounds to me like something in the drive line is loose.
try pulling the wheels and checking the brake backing plates, i have see those come loose just enough to cause a shift when the brake is either applied or released.
It could be something else in the drive line also, so you may have to get someone to operate the machine and watch for any component that may be moving that is not supposed to be, when this occurs. Check for loose bolts on fixed components.
Just listen carefully where the sound is coming from and concentrate on that area, if anything is loose you should see it.