sounds like gear float. when you have a lot of lift and the driveline angle is steep it will make a grumble/rattle noise in your t-case/tranny when you let off the gas.
mine has done it for the last 4 years or so.
dont think it hurts anything, but it gets annoying.
most have cured it by running a c-v in the rear.
if you already have a c-v then it it probably another problem.
check u-joints, pinion nut, flange nut on t-case.
derek