[buchanan]

Check the motion of the wheels when trying to move sideways. One side should be turning inward with the tops moving toward one another and the other outward with the tops moving apart. It is possible to get the symptom you describe by getting various combinations of things crossed up (motor positions, direction of rotation, etc). If they are moving "both inward" or "both outward" it just sits there and chatters against the floor as the two sides work against one another trying to split or compress the robot.