Thule is a Swedish company that makes a wide variety of roof racks and accessories for installation on passenger vehicles. Thule racks are popular due to the versatile designs that can adapt to almost any vehicle, but one negative side effect of mounting a roof rack is increased wind noise while driving. However, wind noise can be mitigated with some effective materials and preparation.
Reposition the rack. If possible, experiment with the placement of the rack on the vehicle. If the rack is attached to roof rails, for example, move the rack forward or back on the rails and test to see if the change in placement leads to a change in airflow over the rack, which is what causes noise issues. In some cases, however, roof rack placement cannot be changed.
Add a fairing or wind deflector to the front of the rack. A fairing fits over the front part of the rack and diverts wind away from components such as the front crossbar and the rack mounts. A fairing may also cover whatever cargo is located in/on the rack itself, depending on the type of installation. Thule offers fairings that are made to fit specific products, but universal fairings are also available that can be adapted to most rack systems.
Use an enclosed cargo box. Most enclosed boxes are streamlined to minimise drag and wind noise. If your cargo can fit in the space, then the decrease in wind noise and drag-induced fuel consumption may be worth the additional cost of the cargo box.
Use duct tape or electrical tape to wrap exposed gaps, joints or holes in the rack system, including the clamps used to connect the various parts of the rack. Wrap carefully but liberally so that the end result is a smooth surface presented to the airflow. This reduces turbulence and noise from the airflow while the vehicle is in motion. Try to terminate the tape ends facing away from the airflow so that the slipstream won't cause the tape to peel away.
When your cargo is loaded in/on the rack system, give it the once-over to look for loose or unfastened straps, bungee cords, fabric/tarp edges or other pieces that may be loose or able to flap or catch the slipstream. If you find anything that appears loose or unfastened, then take the appropriate steps to secure it. This may mean tightening straps or cords, repositioning cargo or using duct tape or similar to "stick it down." This can also help you be sure that your cargo is securely fastened for the drive.
Major retailers can work with Thule owners to determine the best combination of components for specific vehicles and cargo hauling tasks. Road-test your work before embarking on a long trip so you can be certain that your noise-proofing is effective.