There was a GTO with a fresh rebuilt 400 at the shop the other day. Owner was complaining about the engine making noise. Turns out the engine builder installed the wrong spark plugs (3/4 reach rather than 3/8 reach) and the tips of the plugs had hit the valves. The valves got dinged up, the pushrods got bent, the cam got wiped out and the whole motor needed to be torn down and rebuilt since it was full of cast iron dust. Moral of the story is that all it takes is one small thing to be wrong and an engine will start to self destruct.