Sounds like some blowby of the rings, particularly if it is under load when it does it.

Take your oil cap off and rev the motor. You should not notice a ton of air being forced out. Place your palm over the oil fill and pull it off quickly a few times. If it feels like it is really building up pressure in there, and providing your beather isn't clogged and the PCV valve is working, it's coming by the rings.

This may not show up on a leakdown test, either. Ring flutter and other phenomenon that occur only when the engine is under load can't be predicted well with a leakdown test.

Bore, hone, clearances checked? Rings gapped and clocked? These things should not happen to a newly rebuilt motor if all was done correctly.