Oil Pump Noise?

I would be concerned. 5 sec is a long time for engine noise. It sounds like you paid good money to have the engine rebuilt and installed. Take the car back. There are a few ways to check the "no oil" theory. They could pre-fill the oil galley via the oil pressure switch port, using a pressurized bottle (I use a pumped lawn sprayer bottle). That would tell if the problem is from no oil or from warm up. If no noise, look at the stand-pipe suggestions and maybe the relief valve in the oil pump which I think acts as a check valve to prevent drain-back.