Help Identifying this SR Noise?

[burnsauto]

yeah it could still be piston slap with good compression. Over time, the compression could go down as the rings wear more, but since it's a fresh rebuild you could still be getting good compression due to the freshness of the engine.

Try doing a leak down test to further check on things if you don't want to go elbow deep into the engine just yet, just know it's inevitable. Hopefully there is some sort of warranty from your engine builder if that winds up being the case.