I know there was a similar question before but, what are some tips for making my large boat rack on my truck not howl so much. It is a square tube, full box type of rack.
are the tube ends open? If so, put some plugs in them.
Nope there are no open ends
First, the cause should be identified. Is it the tubing vibrating in the wind, or is it a noise due air blowing over some feature including the tubing?
Tubing with holes perpendicular to the air flow can howl like and out of tune flute. This would be akin to drilled holes on the top or bottom of the tubes.
If the tubes are resonating, this is harder to fix. One part of the solution is to move the resonant frequency out of the range of the forcing function, usually by stiffening which increases the resonant frequency. Adding vibration dampening can help. A thick coating of bed liner glop may help. But, I understand that once the liner coating is applied, it is hard to remove, so you may wish to experiment with removable stuff before applying some thing permanent. Tightly wrap one of the tubes with black plastic tape or duct tape. Use several layers. If this damps the vibration, then consider going to something more permanent. (Good ol duct tape. What'd we do without it?)