Jaydee - sounds like the type of crabs ive got.
(In my tank i mean

)
I have two that dont come out - just move around the cracks in the LR, and one chops off bits from around its main hole.
I thought this was all i had - but after my tank cracked and i had to move everything guess what fell into the bottom of the box i put the liverock in for transport?
MANTIS SHRIMP!
It now makes me think i might have more...
Anyways try this - if it doesnt work then you can be pretty sure your crab is the clicking culprit.
Get something appealing like a couple decent chunks of shrimp or similar.
At night, when the lights are out and the room is as dark as possible, drop the bits of food down into the very front of the tank. Sit and wait with a torch at the ready, and see what comes out to eat.
If Mantis appear - shine the torch on em - apperantly this blinds them and they wont move for a short while. Quickly scoop it up and do what you will with it.
If no Mantis come out then you can start breathing again
