First things first:
Type of filter: Rena SMART30 HOB
Problem: The motor sounds like it's straining, even at maximum there is almost no flow, and there is a gurgling water noise coming from INSIDE the filter. Normally this filter is almost completely silent.
Things I Tried/Checked:
-I cleaned out the entire filter and rinsed the sponges. There are no blockages anywhere.
-the filter is level, not tilted
-the water level in the aquarium is level with the output, so that's not where the running water noise is coming from
-I checked that everything got put back in the filter the right way round
-it's not the impeller, if that gets clogged it makes a rattling noise.
Does anyone know what else might be causing the problem with my filter?