Really poor ground location. The bolt hole you are using is actually one of the worst possible grounding areas. This is because the nut on the backside of the sheetmetal is basically tack welded on. Either run a new ground wire direct to the battery or first scrape the paint clean in a new area and make your ground connection at this new point. I hope that you have already upgraded the ground wires from the battery to chassis and battery to alternator as well. If not, these need to be done pronto.
Have you tried muting plugs yet?
What are muting plugs Rob?
Muting plugs are designed to dead short the input section of the amplifier. Take some crappy old welfare rca cables and snip the wire about 3" from the end. Connect the center conductor to the outer conductor (thus creating a short). Make more of these as you'll need them. Plug them into the rca input of the amplifier. Do you still have noise. Yes = problem with amplifier and wiring, No = problem coming from head unit (in most cases but not all).
I'm still betting on the ground as being bad, get it changed first and post up the results.
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Top Secret, I can tell you but then my wife will kill me.