If its like my wife's Malibu with a 3.1L V-6 I would suspect the two transmission mounts as the engine wants to roll when in gear. The inserts were $18.00 for these two mounts a piece at Auto Zone. The back one is hard to get to and the front one is easy after you move the wiring harness in front of the nuts. The front would be the most likely put the car on 4 jack stands. Have somebody in the car start it and put it in gear with the foot on the brake. Use a flashlight and watch each mount as they rev. the engine slightly.