Weird Noise? PLEASE HELP

[95TealCamaro]

Well I'm going insane. I have 2 12's and 2 amps with a crossover. I still have a factory delco cd player (plan to get a new one soon). My stereo sounds great when it's on. But when I turn the HU off and the ignition and battery are still on, the 12'2 turn on full blast with a vibrating sound (it is sound ungodly loud, it rattles the crap out of all my mirrors). The only way I can get it to stop the noise is to turn down the sub gain and the x-over frequency on the cross over. I know this is wrong and I can't figure it out. I thought it might be the ground (It is connected to the rear seatbelt under the panel) because I hit my rear seat today and it made that noise through the 12's. I don't understand....I mean it works perfectly while I listen to music just when I turn it off. I took it to a shop and they said I need an aftermarket deck because the line output on gm decks are doing that. Another shop said it could be the ground or bad wiring. I am so frustrated and pi$$ed off. Can anyone help me. PLEASE
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Thanks