Any guesses on what's making noises in my tank??

Every night at around the same time, probably about 20 minutes before the lights bed time something it making a noise in my tank. It almost sounds like if you were to rub to wet rubber things together, that squeaking sound. It does it about 3 times and then stops and then 5 times and stops. It comes in bursts.

Last night I watched every one of my fish and none of them were doing anything weird. At one point I figured it was the fox face trying to chew on something.

Here is my stocking list
5 peppermint shrimp
1 Cleaner shrimp
1 candy striped pistol shrimp
1 porcelain crab
5 trochus snails
1 fox face
1 melanurus wrasse
1 diamond watchman goby
1 blue spotted watchman goby.

:confused:

If I remember I'll try to get a video of the sound.