When I'm at speed--say, 80 or so on a freeway--I've noticed that the hood shimmies a bit. Just keep an eye on the panel gap between the driver's side front fender and the hood and see if you don't notice it, too.
I have a notion that whatever air is getting through the hood scoops (I've left the rubber inserts in place) is causing a bit of lift under the hood, making it vibrate a bit. I'd be curious to know if '04s do the same thing, or if their hoods sit still.
Anyway, I'd imagine that taking the inserts out would only increase this phenomenon in steady-state cruising at speed.