Other than a custom full length rack with a fairing, an ikamper and a cargo box.....nothing!
Well I'm gonna go out on a limb and say the wind noise isn't from the van design then. Lol. Best of luck getting the rack and gear to be aerodynamic and quiet. I need to make a small deflector for my leading solar panel and hope that cuts the wind noise. Only other option is plenty of kilmat and insulation in the walls and ceiling.