Michaelmah I went and listened to my tank after i read your post and sure enough i could hear the noise too, I think i have figured it out though.
I noticed that the mystery noise correlated exactly with the escape of bubbles from my crystal CO2 diffuser, i don't know why they would but i believe it is the bubbles making this noise. Do you have a CO2 diffuser in the tank? if you do try turning the CO2 off and listening for the noise.
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