I bought a gx470 last week, i am noticing the same problem.
just over 55 mph, i could feel lot of wind turbulence noise...
The noise level ofcourse goes down a bit if i let air recirculate....
It certainly doesn't feel like a window sealant problem.... ( i mean wind noise is
not focussed but generally coming from all around the vehicle windshield area... )
The service guy who took it for a ride, did admit to the problem, but says don't
really know what to do...
I also checked whether the luggage racks are pointed correctly....
Is there any specific soundproofing i could ask them to look for....
I called the lexus customer support line, they claim they don't have such a problem reported...
Otherwise i am generally happy with the rest of the vehicle.
-jlogan