When I saw your post I immediately assumed it was a problem with the laterals in your filter. If you're not extremely careful adding the sand back to the filter you can crack or break the laterals. But what a pain to have to unload that thing again to check them. If you do end up going through the trouble of emptying the filter back out to check them I would strongly recommend going to zeolite. It's a much more effective filter media and it is lighter than sand so far less risk of damaging the laterals when adding it. Good luck and let us know what you find out.