A4 - Annoying Wind Sound?

[Justin Fidler <justin@codeboy.net>]

One thing I've noticed about my A4, which occurred again tonight when a
passenger put down the rear window.  When one (or both) of the rear
windows are down about halfway, and the car is cruising at least 25mph, it
has this really loud reverberating wind sound -- like helicopters are
nearby. It was so loud that my passenger promptly put her window right
back up. Has anyone else had this?  I've got the 1.8t engine and the sport
package. It's got to be something with the aerodynamics.  The front
windows were up at the time.  I didn't get a chance to do more testing.

Justin
97 A4 1.8tq