You, were wandering the infinity castle at the final battle, killing off demons

Until a small rumbling noise could be heard above you.. heading down.

The rumbling noise didn't stop, it just got louder. It sounded like a demon was falling down through the roofs, and would probably land soon

It was probably a upper ranked demon, looking for a demon slayer to kill, since it was the final battle. The battle to kill Muzan kibutuji.