What is the knocking in my 5.0 engine?

There is a knocking in my engine. It does not overheat nor run any differently, it is just really loud and annoying. I rear ended somebody a few weeks ago and had to have the frame pulled out and a new belt put on but the car still runs. The only difference I have noticed is I used to be able to spin my radiator fan with my hand but now it is completely immobile unless my engine is on and then it works normally. What is my possible problem and how much to fix?