My partner works for Toyota and has done for many years. This is a characteristic of the vehicle, because they idle so low. It is not going to cause any damage to your car. You can try to turn on the rear de-mister, in the past it has helped to bring up the idle a little bit to stop the vibrating. It puts more load on the alternator causing the computer to bring the idle up slightly. He has worked on hundreds of them and they all do it. Some more so than others. I hope this has helped. :D