this probably won't be much of a help, but i have installed marine carpet in my rig. i can't tell you anything about the sound levels, b/c my motor is still in the machine shop, and my truck is still in the middle of a total build. however, i sprayed the bed w/ rust preventative paint, then layed down regular carpet padding, keeping it a couple inches shy of the walls, and then layed down my marine carpet in such a way that water cannot run underneath it. i'm hoping that the padding will provide some head/cold insulation as well as protecting my wife's ears (i wouldn't mind listening to the loud rumble of the 6.2 i built all day long - but it's another story for my wifey.) anyway, just for what it's worth. i picked up my carpet at lowe's for like $4 per linear foot - which = 6 sq. ft. per $4.
