There are several things to check if the unit is making a vibrating or buzzing noise.

• Remove the pump and make sure the impeller is clean and that the air intake is clear.
• Check that the pump is assembled correctly and that the impeller assembly is moving freely in its bearings and undamaged.
• Check that the plate sitting behind the pump is all the way down and secured in to place with the fixing screws.
• Make sure there are no cables or pipes vibrating against the body of the protein skimmer.
• Make sure that the skimmer itself is securely in place and not vibrating against the tank.