fish

I agree you should check the water level. full fish tanks don't make alot of noise. I have a 90 gallon tank with a 2 ft algae eater and a 31 inch pacu and I pull my filter and clean it every couple of weeks. A couple of months ago my nephew was watching tanks while we were gone. He never cleaned the filter and I was home a week when my water was making alot of noise with the tank almost full. It was because my filter was running at about half, and the water coming down was being disrupted by an over-full sponge. Just another thought.

If you have a full cover , it will cut down on evaporation so that the water will not have to fall so far (but make sure to do regular weekly partial water changes). The full cover will also help to keep fish from jumping out and other things from getting in. Since it uses electricity it will make the electricity bill go up some but not much. Make sure to run the filter 24/7 or you might kill the good bacteria and then the fish. The good bacteria in the filter need oxygen to live. If you shut it off there will be a lack of oxygen in the filter. Also if you shut off the filter and you are overstocked it will cut down on the oxygen that the fish can get and will starve them of oxygen, so they will come to the surface to get the oxygen that is being drawn into the water at the surface.

You sure it's only the water you can hear and not the pump?

if your fish is a fresh water fish then you can turn off your aerator at night but if your fish is a salt water fish then its a problem because your tank does not have any plankton