The clicking noise is an extremely powerful super-suck!!! they use this to extract the juicy bits from snail shells which correct me if I am wrong is one of their main food sources in the wild. If you put some in you may get to see them do it, I have only seen them do it on empty ones they have already eaten, I suppose you would have to sit up all night to catch them doing it.
PS be careful adding snails to your tank.
Mark.