Not sure why'd you split the cases to look for a knock. Pop the left engine cover off and try moving the alternator up and down, then in and out. Main bearings will let you know they aren't happy. Remove the right side and look for the usual cam chain stuff, and try the same wiggling on the right side of the crank.
If you are still at a loss, you can pull the top end, hold the crank steady and try moving the rod up and down.
Look for something loose somewhere.
If you are lucky, Dave is correct