My 1997 manual 2.5l outback has some shaking features. Whenever I am driving in about 3rd or 4th gear and between 3000 to 4000 rpms it shakes a bit. It's not all the time but some times it's bad. My motor mounts are needing replacing so I know some shaking is from that. But i know not all is that. What else do I need to do?