Be sure there is water in the pump chamber -- if you don’t add water to the pump chamber, the filter will pull in air. Also, take care with how much water is in the aquarium. Most filters need the water level to be about one inch from the lip of the filter, and a shallow tank may prevent the filter from working. Finally, make sure your bubble walls and/or air stones are not directly under the intake tube. If bubbles go up the main tube, it will cause a rattling sound and may stop your filter from working.