It's the surface. It's hard to explain, but if you were to go to the Salt Flats currently, you would understand what I mean. There are little bumps and ruts all over the place. At a slower speed, they are more prevalent. Once the MS3 hit 130, there was enough air flow under and over the car, that the attitude of the car created a skim effect, thus making the car seem to float over the surface. The big difference with our cars at speed and streamliners at speed is the ambient wind co-efficient.
Streamliners have a rougher ride due to the fact that they are going so fast, they are getting buffering from cutting through the air, and are purely downforce driven, very similar to a fighter jet before and after the sound barrier.
There is no way to really describe it, rather than to live it.