Don't get discouraged... I recently had a similar problem with mine and everything now seems fine. I ended taking mine apart to double check everything and found nothing. Eventually found the problem was an eternal part. I wish I had taken time to go over everything carefully before I took it apart.
Start by making sure everything is tight. Standard or automatic? Could it be something loose on the back of the engine like a flex plate...or something slightly out of line? How about crankshaft end play? Try your best to locate where the noise is coming from. And if you have to open it up again, it's a pain, but it's not the end of the world. In that case check all your clearances again. You put it together the first time, you can do it again.
Good luck.. Others will have suggestions as well.
