Loud metallic knocking sound

Lost oil pressure on break in with remanufactured Ford fe 360 engine. Replaced cam, lifters, springs, etc. This break in went better but now have a loud metallic knocking sound that starts around 1,200 RPM's and gets louder with higher RPM's but the knocking stops 3,000 RPM's. With stethoscope I hear the knocking the best under engine with pointer on bottom of engine block in front of inspection plate or at back of oil pan. I have feeling Rod Knock, but confused why the knock stops 3,000 RPM's and not under load this is happening. Yea, I drove in back yard a little with no knock until you give it fuel. This is a 4 speed transmission. Have heard dozens of could be knocking noise over the net, but never asked a mechanic until now. Please help, much appreciated.