If you have an aquarium air pump that makes a terrible rattling sound, you can modify it with a few squirts of hot-melt glue, silicone or any other space-filling adhesive or sealant to be as near to silent as it's ever likely to get.
I have two Rena Air 600 air pumps (it's called a Rena Air 400 in the US), and they were both really noisy until I opened them up and modified them to stop the components inside from rattling around.
Wrapping air pumps in towels will muffle the sound, but quickly leads to overheating of the pump, which shortens it's life.
