If I was a betting man I would wager that you have a broken piston skirt. Pull the head and oil pan, find the bad piston and replace it. If it doesn't smoke your cylinder walls are probably ok. Drop your pan one weekend and see if there are any surprises in it, and you can see your pistons. Also you can take a timing light and gauge whether the sound is top end (lifter) of bottom end (piston, etc) by seeing whether you hear a knock per flash (bottom) or two knocks per flash ( top)