I have a couple of Pondmaster pumps that would start to rattle and I would put in a new impeller unit. Now they are "scoring" the impeller magnet and only run for a short while, maybe 4 hours, before stopping. In cleaning the pumps I notice the impeller channel is scored on the inside. Is there anything that can be done to fix the pump? New impellers just seem to rattle around. Also, any experience on the cause/prevention going forward?