Mine had the same noises.
That noise (every few seconds) becomes because the pump's performance is too high. So reduce the speed and the level should remain the same (there will be a kind of gurgling sound, but it sounds constantly, not every few seconds). Then put the sponge back.
What I did to silent my Aquaview: at first I removed the sponge and balls from drain pipe. Then bought 32mm pipe (original is 40mm, but the hole is no more than 1 inch) and fittings for that, also ball valve (not very accurate way, but it works somehow and moreover I can change the speed of pump), and also Maggie Muffler silencer, now it's quiet.
