that is your ground for sure. you either have a bad grounding point or bad ground wires.
if you have a multimeter you can check continuity at the grounding point to see if is sufficient.
when you have your radio on and the amp is on and you hear a rumbling from the subs that is because your ground is not good. it is causing your amp to become unstable. you can ruin your subs or amp.
if there is a whine type of sound then chack your rca wires, make sure they are not touching or close to any other wires