My guess is that this is from the fish care function, every 20 seconds the prop should turn 1-2 revolutions to scare away fish. This could result in clicking if the drive unit is worn or very old and the prop as some free play on the magnet/plastic shaft it clips on. It should be tight on the shaft such that you can only turn it on the shaft with some force, it shouldn't move freely. It may also be the magnets are on thinner material and this compresses the rubber feet so noise is transmitting to the glass/acrylic more than it should, that would explain why moving the magnet helps. If the material is 1/4" or less you might try a pad of some rubber between the inside magnet and glass/acrylic. The other option would be the 6025.650 silence clamps, these are new and prevent the vibration of the pump from reaching the magnet and glass.
