WiLL
I have seen some of your posts before and know that you are somewhat proficient in the car audio world. It sounds to me that there is a grounding issue but it is most likely is in your headunit. As much of a pain in the ass it may be, pull out your headunit and check the wiring behind. While you have your headunit out and plugged in, turn the volume all the way down and give the wires a gentle shake. If you here more of the rumbling in the speakers, shake individual wires (e.g. ground or RCA) and look for the noise again.