Engine rattles and idle

My engine vibrates and makes rattle noise at idle mode. When I drive no noise,but when I apply the braked at a full stop and sit with my foot on the brake peddle to keep it from moving such as at a stop light, it vibrates so hard that it shakes the whole car.The same situation at parking mode when the engine is running. I have been told the motor mounts may be going bad. How can I tell if it is that? I think it may be either something loose from when they replaced the proportioning valve since it wasn't doing the noise until they replaced it.