We had the same problem with our 1999 Sienna. The problem is with the rear springs, and the Toyota dealer fixed the problem while the Sienna was still under the new car warranty by adding rubber pads to the rear springs. We had taken the Sienna to the dealer numerous times, but the dealer was never able to duplicate the problem. Finally, the service representative attended a Toyota seminar where the problem and a fix were identified. After the rubber pads were added, the "boing" never happened again.