How long has it been since you last changed your oil in the differential? That is supposed to be done every 15K, according to DC. If it has been a while, do the oil change, and inspect the gears inside your differential. Not unheard of to loose the end of your axle, the button, into your diff. fluid. Some one on this forum just had that happen a couple of weeks ago. Also check your U-joint at the end of your rear drive shaft, it may be going south on you. Releasing the pressure when putting in your clutch, causing the noise to stop, may just be the U-joint ready to break.