Figured this out. What had me puzzled was why the crossbars' orientation would matter in terms of noise. It wouldn't, unless...(light bulb goes off)...they're shaped like an airfoil (wing). And they are - thicker at the front and tapering toward the rear edge.



The key is how air flows over such a shape:



When the crossbar is installed correctly (thick side toward front of truck), air flows smoothly and it's quiet. If it's backwards, with the thin sides toward the front, the air flow is very turbulent and it makes the howling noise. I reversed mine and just got back from a nice quiet test ride.

The one that goes in front has a 0 embossed on one end and a 1 on the other, and the one that goes in back has a 2 on one end and a 3 on the other. The thick side goes toward the front of the truck. You'll know it's right if the tightening knobs are all on the "outside" (on the front of the front bar and the back of the back bar).

Also, if anyone is considering repainting their rack, Rustoleum's Black Stainless Steel metallic looks great.