how come when i connect my zune to my aftermarket car stereo, does it

make a humming noise with my driving.? the headunit is a valor 603w touchscreen
it doesn't making a humming with the FM radio, it only does it when i have the zune turned on and connected to the rca aux. inputs in the back of the headunit. theres an amplifier hooked up to a subwoofer also, but that never made any humming noise. the stock stereo didn't make a humming noise either, it was connected with a 3.5mm pluginto the front of the stereo to another 3.5mm plug into the zune jack. The aftermarket stereo is connected with a 3.5mm plug connected to the zune jack then it splits to a the red and white rca plugs for the aux. inputs in the back of the headunit. I'm not sure if its just a grounding problem or the cable it self? Any help would be perfect. Thanks.