Weird clattering sound, Carbed 1.7 is clattering, but compression checks out. Options

[LetoAtreides]

So, I just completed my first successful engine swap from my blown '75 1.8 to a new used '72 1.7. However, after firing up the engine, there is a strange clattering sound. The valves are adjusted perfectly, the distributor, wires, plugs, and timing all check out correctly. Compression also checks out fine. It doesn't sound like a thrown rod or a bent pushrod. I also don't think it could be a foreign object in the case or cylinder, as the sound is only present when at low RPM. One revved, the sound goes away (as opposed to getting louder). My friend thinks it might be the carbs (dual Dellortos) running rich, and my dad speculates the oil might not be circulating properly, because the oil I drained from this new (used) engine before putting new oil in was pretty black and could be clogging up the oil screen. Any thoughts?