Does it sound like this?
http://www.geocities.com/ma71supratu...h/rodknock.mp3
Keep in mind that a spun bearing can sound differently in different cases, not to mention that sounds in life may sound different than what you would have from recordings and replays.
If you have a spun bearing, you may see shavings in the oil.
That would be one way of diagnosing it; check your oil after you dump it.
Another way of diagnosing it is to remove one spark plug wire at a time to see if the sound goes away or changes greatly. Removing a spark plug will decrease the cylinder pressure and therefore decrease the pressure on the piston, which in turn is tranfered to the rod.
Another way of diagnosing it is to spend a whole ten dollars and buy yourself a mechanic's stethoscope as sold on eBay, then learn how to interpret what you hear.
First do the diagnosing, and then I'll tell you more if need be.