The unit might too be powerful for your size of aquarium. You can fit an Air Valve which will reduce the flow.
We would recommend checking that the unit is not vibrating against something as this is often the cause.
Check that the diaphragm is not damaged - if it is, then you can buy a spares kit, which contains everything needed to maintain the pump.
A check valve is fitted in the silicone airline and allows air to flow freely to the aquarium, but prevents water siphoning back into the pump if there is an electrical power failure.
We would please ask that you could conduct the following checks:
Check the airline and air stones for any blockages or damage.