Try taking 1-3 rocks out at a time. Remove the rock(s) and put them in a bucket of tank water. Then listen for the clicks in the tank over the night. If no more clicks then you've got the rock(s) you want. If you still have clicks, remove another group of rocks.
Though you may find the animal that is clicking, it may still not be the thing killing your snails. I'd still use a video recorder if you have one.
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