It could be you have a bad mount, and when you try to drive away before the motor has "torqued over" against the slop, It pitches the motor suddenly, and you feel the shudder and hear the impact. Try putting it in gear with the hood up, and watch what happens when you hold the brake firmly and give it gas in gear. If a mount is broken, you will see the motor pitch over hard, and may hear a thud along with it.