To reduce noise from from the rack caused by airflow one must think about the airflow of the particular vehicle and get the front of the platform out of the main jet stream going over the vehicle.
When used on the back of a truck, the air comes over the cab and then takes a hard dive into the bed off the flat back of the cab. It’s putting the front perimeter right smack in the jet stream. Raising the platform an inch or two and or pushing it back slightly will likely make a noticeable difference but it will never not make noise.
On the roof of a vehicle we have found through our testing that moving the platform fore or aft, or right or left even an inch can make a difference.