"HELP" Rapid tapping noise freaking fish out

[salty210]

Hi; I am hearing a strange knocking noise coming from inside tank. Have turned off all pumps and I still hear it. It sounds like tapping your finger on a table. Sometimes its only a couple of knocks and sometimes its up to 7 or 8 rapid knocks. No specific time when it happens, lights on ,lights off, feeding time, or when ever. It drives me and my fish crazy. Any ideas what it could be.

210 gallon aga
50 gal sump
vertex 250 skimmer
200lbs live rock
4" sand bed
8" blue face angel had for 6 years
5" powder blue tang
4" blue cheek trigger
7" foxface rabbit
4"threadfin butterfly
coral beauty
7 green cromis
cleaner wrasse
long spine urchin
sand starfish
40-50 hermit crabs some quite large
30 or so snails