Broken part
During the last months I have thought that the flow has not been as good as I use to be, even though I rinsed the filter it still did not result in a “normal” flow, and also I think the pump has been a bit more noisy than normal.
A few days ago the filter stopped al of a sudden, I thought it might had gotten to warm as it had sucked in air from my surface skimmer, and all of a sudden it started again and I thought there were a heat fuse or similar.
Later it stopped again and this time it did not start again and even though I waited all night it still did not start.
When I took the filter apart I found that the magnet for the impeller was broken in two, and were pressing against the sides of the pump housing, I managed to push the pieces together again and the pump started up but with a slightly high hum.
Yesterday I managed to find a shop who had a new impeller in stock, the impeller is exchanged and the pump is now running as good as new.
The bottom line is that you should be careful with air / gases entering the pump as it is a risk that the magnet is damaged when it rattles and banging in the housing, this is an unnecessary cost and a risk for the fishes.