Single or dual batteries?
If dual, aftermarket or OE wiring?
If dual OE wiring, remove one of the batteries and try it again.
Both of my Sub's did this with the dual OE wiring. As best as I could deduce the problem is in the grounding points not being the same exact spot. Going to using a marine battery bank switch on the ground side forced them to ground at the same location and cured the problem while also allowing me to turn either battery off.