speaker whine?

I know usually these things come from a grounding issue but maybe someone can point out my mistake. I added 2 6v to my dual 12v system for amps...I put the 2 existing 12v's on perko switch 1 and added the two 6v's to perko switch 2. I wired the grounds on the batteries banks together with 4awg. The system has a pretty audible whine only when the engine is on, then goes away if you turn of the HU or turn off the boat engine (with the stereo still on). Did I miss something in my install? Fyi the boat is brand new and worked fine before the addition.