The same thing is happening on your roof rack.
Thule roof bars noise.
On some cars manufacturers have gone to a radio antenna with what looks like a spiral wire wrapped around it.
The new aero foil cross bars provides a significant improvement in roof rack noise.
They are noisy and when it rains they whistle especially when bike carriers are attached.
The design keeps vortices from slapping back together after the antenna has sliced through them.
All roof bars will make some noise.
You can also wrap some 1 4 to 3 8 rope in a spiral around both bars.