Aquarium air pumps operate via a motor that causes a rubber diaphragm to oscillate at high speed, producing a steady stream of air. As the diaphragm oscillates, the vibrations are transferred throughout the pump. While most air pumps are not whisper quiet, even when new, they are designed to absorb most of the vibrations. As the pump ages, the constant vibration causes wear, leading to excessive noise. There are several things you can try to make your fish tank's air pump operate more quietly, including checking for loose screws. Clogged air-stones can also restrict airflow, causing too much vibration.
Check the rubber feet on the bottom of the pump for damage. Replace any that are cracked, broken or missing.
Examine the mounting screws that hold the rubber feet to the base of the pump. Tighten with the screwdriver if necessary.
Disconnect the outlet tube from the pump and remove the air-stone from the tank. Place the tube to your mouth and blow. Replace the stone if it proves difficult to blow air through it.
Place the air pump at a level above the tank's waterline to reduce the load on the motor.
Move the pump to a solid surface if it's resting on a hollow structure.
Set the pump on something soft to absorb vibration, such as a folded washcloth or piece of foam rubber.