The source of the sound
The party's search turns up a tarnished silver holy symbol and a small red gem of some sort. Assorted silver and copper coins seem to be scattered about the lair, no doubt secreted there by the rats. There is also a broken metal point of some sort, separate from the arrows that the party fired. It perhaps came from an arrow or spear.
Before there is time to properly count and assess the coins, the party hears a faint whispering sound, like a distant exhalation. The scraping sound is heard again, much closer. To your horror, figures round the corner, stepping out of the gloom at the back of the cave. They stride towards you with deliberate, exaggerated slowness.
OOC: Roll Initiative!