Not a rena owner but that would sound like an impeller scraping the side of the well. My filters would have a rubber bushing on each end of the shaft inside the imepller. Perhaps look to make sure those are in place and that the shaft is in the bushing,, not sticking down beside it on one end? Another point might be a worn shaft or impeller if for some reason a bit of sand get into that area. Assuming this is a scraping and not an air leak that is misleading me?