If it only knocks when you take off or are moving, and you don't know for sure that it is an engine knock, check the u-joints on your intermediate drive shaft. I came very close to having a disaster because I didn't know to check those joints, and I had never heard any other vehicle make the same kind of noise.
The scary part for me was the fact that the first time I crawled under and tried to wiggle that shaft, there didn't seem to be any slop. A couple of weeks later when I checked it again out of desperation, there was mucho slop. When I went to replace the joints, I found the needles in two of the arms on each joint gone, (as in not there at all). I still don't understand why it didn't move the first time I checked it.
Mel