Would that be a bzzzzzzrrrrrr or a whirrrrrrr or a clankity clank clunk sound?
Sorry hard to figure out what you are describing. Is it possible that you have a powerhead on same timer as lights and are hearing it click off and the impeller wind down?
It's actually possible it could be a fish or inveretebrate. I have a pair of coral catfish (plotinus lineatus) and they will make a 'buzzing' or whirring sound periodically. Audible sounds are not relegated to just these fish many other species can make them.
what kind of fish do you have? my domino damsels used to make a purring sound. and my angels occasionaly talk to one another with more of a clicking sound.
I dig my pistol shrimp at lights out. He must get spooked or something because he will snap at least 5 times every time the lights go ut. After a couple of minutes, he stops....strange..