I own an '04 silverado,4WD, that seems to make a popping noise like a chain dose under tension, only when I'm in 4-low. under further inspection I've noticed it coming from the transfer case. when you hit the gas the front drive shaft torques a little but dose not turn,but the rear turns, and thats when you hear the popping noise.I'm not real tech savy, so any info would be great.