Hello engine builders, should I be concerned? I just started my freshly rebuilt SBC 400. Motor has a new Dart SHP block, new cam, rings, bearings etc. I had it rebuilt by a reputable machine shop who only builds racing engines. I paid extra for the engine dyno and everything performed perfectly during testing. My issue is that the motor sat in my garage for about 8 months before putting it into the car and finally, I got it started 4 days ago. I left the original oil he used in the oil pan during testing and figured I would replace it after getting the motor started. Anyhow, on startup for the first 5 minutes or so you can hear a knock coming from the passenger side low in the crank area, but it goes away once everything warms up. The engine has no miss fire, it does not heat up and it pulls hard when revving the engine. Once it heats up the cam rumbles nicely, no exhaust leaks, 55lbs of oil pressure on startup and then settles in about 25-30 lbs after oil heats up, actually engine sounds great once it's warmed up. However, on every occasion, when cold, the engine knocks for a bit and then it goes away. Yesterday, I removed the oil and found a marble size buildup (oil and I guess very fine metal material) stuck to the magnet of the oil pan plug. I took a paper napkin and wiped it off. It just smudged onto the paper towel, nothing solid. Is this part of the break in ware or should I be concerned? This is my first rebuilt motor and I am not sure what to expect. My engine builder said I did not have to adjust anything on the motor, valves, carb, etc.. everything was adjusted on the dyno. But, why would knock like when it is cold. My previous motor with similar build, never did this. Anyone have any ideas?