If you are hearing a knocking coming from the bell housing, your damper plate is probably in need of replacing. The part itself is rather cheap, usually under $300. It's the labor of swapping it out that gets you. I was quoted close to 4K. I got more time than money, so I went in head first to change it. You don't need to pull the motor with this method. In this video you'll see some of what goes into it. If you have any questions, I'll try to answer.