The same thing is happening on your roof rack.
Thule roof bars noise.
The thule aeroblade and rhino rack vortex bars both feature noise reducing strips that makes them almost silent in many applications.
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.
On some cars manufacturers have gone to a radio antenna with what looks like a spiral wire wrapped around it.
The thule bars are cheaper and with money being tight and all.