Totally normal. Its not the airbox, its rather that it is a very open, large air filter and box so emitting all the sounds from the engine which what you here is the clatter and natural resonance of the valvetrain at those RPMS' and gears.
Love the mechanical sound of this car!
The one thing is you really never should be in a situation where you are accelerating in this rpm range in a high gear. You need to keep this thing about 5k rpms so it is never an issue in real life driving, atleast for me!