try taking off the splitter and just hooking up the rca's to the mids/highs amp
mine did it when it was on only with the engine, off it didnt make any noise
another thing you can double check your grounds on the deck
what i did to fix it on my deck was to ground out the rca's like this....
(google pic)
http://i22.photobucket.com/albums/b342/Ultimate157/PioneerFix.jpg
(mine)
this is the noise it was making, with this one turned out to be the rcaharness on the back was loose(the small pins)
[YOUTUBE="vfvdfvdvd"]ibeOu8j0Me4&list=UUVRoYUPDxc1EJNbol47GRwQ&index=12&feature=plcp[/YOUTUBE]
its just a matter of trouble shooting, because theres a few different things that could cause it, these are the most common so hopefully one of them helps ya