Pull the spark plug wires out of the distributor cap one at a time and see if the noise changes when a particular cylinder is not firing. That one sounds a lot like a broken piston skirt letting the piston slap the cylinder wall as a spark plug fires. Killing the ignition on a broken piston will seriously diminish the noise, or turn it into a double-knock sometimes. Does the noise change as the engine is revved up just a little? You really need to be monitoring oil pressure, even if it's just a gauge connected to the side of the block under the hood. The oil pressure port is a 1/8" pipe threaded port about 2/3 of the way toward the back of the block on the driver's side, about 4 inches above the oil pan rail. If a gauge is not connected, it might have a line pinched off or have a plug in the hole.
Jerry
