Maybe this will help. These are measurements in a car with the windows open.
Here are two examples of low frequency sound levels while in a car with the windows down moving at 55MPH. The microphone was in the middle of the car at ear level. As you moved the microphone closer to a window the measured sound levels went up.
First measurement, all windows down in a BMW 4 door sedan. Here the noise is above the threshold at 11Hz and up, there is substantial low frequency energy present even at 55MPH.http://eminent-tech.com/avsimages/ca...sdown55mph.jpg
We have all heard this inside a car. At speed on the highway With one rear window down we get a strong low frequency flutter. In this case the cavity of the car makes a very large whistle 112dB @ 18Hz which is way above the threshold and clearly audible.http://www.eminent-tech.com/avsimage...wonly55mph.jpg
Obviously convertables and motorcylces would be louder. Trucks, buses, and airplanes would exibit higher levels of interior low frequency sound. Also if you go faster the noise level goes up. There are plenty of examples where we can experience high level very low frequency sound.