I noticed today that while operating my vehicle my engine jerks whenever I accelerate. For example, when I come to a red light or stop sign, and pause for a few seconds, as soon as I take my foot off the brake and acelerate my vehicles speed up to about 20 miles per hr. the engine jerks back; it sounds like a loud thud. But once it is in gear I don't have this issue anymore. It only occurs once I bring my vehicle to a complete stop. What exactly coulod this be, and is it something major or minor in terms of being repaired? This is my only vehicle.....Please help!!!!