It could be the screw that holds the bracket for the air intake silencer.
Here is a schematic:
http://www.aquacave.com/Manuals/CORALVUE-SRO2000I-instruction-manual.pdf
Great skimmer but it is very sensitive to water level. I am happy with mine.
To make mine discharge under the water (quieter), I put a spa hose and a 90 degree pvc pipe fitting with a couple of holes drilled in the 90 degree fitting for air venting.