i smiled when i saw this, because i know the answer...
adjust your gain on your amp... and if its not as loud as you would like it then try this......
i know the 3rd gens have a premium sound system in the, that means there is an stock amp powering the speakers that is not build into the head unit..
but the bad thing for you is that you have to remove your head unit to get to it .
once you get your head unit out, follow your wires from the aftermarket wiring harness to a solver box about 2 inches by 5 inches. i believe there are 2 screws holding it there... once you take the screws out, unplug the wire harness. then (i believe you will need a new wire harness) to plug into the one that plugged into the back of the amp.
that extra amp was causing to much interference and is causing the buzz from the engine
just wish someone could have told me like this so i wouldn't of had to figure it out for my self... GOOD LUCK and if this dosnt make and since, let me know and ill try a post pic of some of the parts i am talking about