Wow - last post in 2009. Anyway, seriously have the same problem as other posters and pretty upset. Noticed it happened far less when I had the air exchanger on just recycling air inside the car rather than from external air. Even with fan off, I could flip back and forth between internal and external air and it would vibrate off and on accordingly. Though not perfectly, but significantly.
Could it be something in how air is coming into the dash from outside?