rattle? the impeller is banging around in the "housing. I had that happen to my peguin bio wheel impeller, I had to pussh it back to gether ( like pinch the end towards each other) due to the removal of it I had started to pull the impellar assembly apart making it longer that the housing would allow for it. Some impellar assemblies have a spacer washer on the ends that has been know to disappear on some brands. Check that too