Why is the motor of my canister filter making a low, rumbling noise?

Noise originating from the motor suggests that the impeller assembly needs to be replaced. As the only moving part in your filter that operates 24 hours/day, the impeller does eventually wear and is therefore not covered under warranty. Also, verify that the filter has been properly primed (always ensure the filter output is above the water surface when priming), as this will help ensure trapped air is not the cause of the noise.