It could simply be a number of things. Lifters, Thrown Rod, bad mains. The lifters caused my Knocking noise and was a pretty easy fix.
Read this thread, it is a bit long but in it you will find the quick and easy way to fix lifter noise:
http://www.automotiveforums.com/vbul...d.php?t=361285
If it is a Rod or Main bearing, it is a very expensive fix.