I had my trans off a couple weeks ago. In the course of reinstalling it, a significant quantity of fluid drained out the back. I went and bought some more and replaced approximately the amount of fluid that was lost. Since then, my trans vibrates/grinds a little when I shift slowly, and also around 3k RPMs. I put the car back up tonight to add some fluid, and when I removed the fill plug, a decent amount of fluid drained out. Did I overfill it the first time, and that was why it was having problems?
