The background clicking noise you hear is probably made by tiny shrimp. It is quite loud at times. If you watch for when the Parrot Fish bump into the coral, you can here the crunch as they scrape off some coral with their teeth to get the algae that is growing on the coral. This algae is their primary source of food. They excrete fine coral sand. Parrot Fish are the source of all the sand on all the coral beaches.