clicking sound in tank

[niloc16]

i've actually had the same problem for awhile and by fluke i heard it while i was at the tank. it was a turbo snail on the glass at the corner and switching to the other pane of glass and it was his shell hitting the glass at it was swinging over. i also it it again when 2 turbos were trying to pass each other on the glass and there shells were hitting the glass