New 408/727 Vibrations

We had a motor come here from a car that had the same issue. It was making a loud knock when installed then it went away. It also vibrated. The knock was the heavy metal that was welded in the counterweight of the crank for balancing hitting the back rail on the block. The slug of nickel fell out and that was the vibration. That was the second one that I saw this happen to. Something to check.

Or could it be a dead cylinder your feeling?