What would cause a clanking sound from the rear of the engine over 2500 rpms to a sputter deleration. The noise is so loud it almost sounds like a nut flying around in the #7 combustion chamber. I poked around with a stethescope and the noise seems the loudest at the rear of the block. I took the valve covers off to check the rockers, rods, and to see if the keepers were still in the spring retainers. What do you guys think? Could this be an exhaust problem? I listened to the collector on the drivers side and it sounds bad as well..