DIRT! As in dross or other type of dirt that's trapped under the lead, flowing out with the stream into the mold. You'll have to do some serious scrapping while fluxing to get it knocked loose from the sides and bottom of the pot. Once you disturb it while scrapping, it'll float out to be skimmed off.
Tapping or vibrating the mold during the pouring process normally takes care of it.
You most definitely do NOT want any vibration of a mold that's being filled with molten lead. Vibration will cause the lead to move away from the inside surfaces of the mold,(as it cools/hardens), causing undersized bullets. Air being trapped in a mold is caused by blocked air vents, or undersized vents. Undersized vent lines can be fixed by deepening them with a scribe or engraving tool. Poor fill-out of bullet bases is caused by a too tight sprue plate.