The only times I hjave had the chime go off while driving has been when my wife unbuclked the passenger seatbelt. In that case it started out as a slow repeating chime and then went faster and faster.

I do not have any rattles at all in my HiHy. Could it be the retractable screen that covers the back area? Usually on the freeway the road is smooth so it probably wouldn't be something loose. Thinking of things that could be affected by wind speed, the only things that come to mind is the roof rack (open the window and see if it gets louder) and the spare tire cover under the car (see if its tight).