Yesterday on the road at about 70+ MPH, we noticed a very annoying low frequency noise on our 09 Unlimited X with the factory soft top. It was one of those kind of pounding noises that you feel as much as you hear. We pulled off the road to check & didn't see anything loose.
We continued on down the road and found that the noise was directly linked to the Recirc position on the A/C. With recirc on, we got the noise. Turn the Recirc off and all is well. Weird.
I would have to guess it has something to do with changing the pressure inside the vehicle due to the change in air flow with the Recirc.