Something to consider.
When first fitted my rack howled a bit at over 50mph. To combat this I wrapped rope around the front betwen the top and bottom, making a sort of spoiler. This worked perfectly and now it does not howel at all. Rope is good becuase you can separate the rope to get the hoolks of load straps onto the bars so it has no impact on use (a metal spolier would have stopped me clipping on the load straps)