wont be the weather are you using a booster pump to boost the water and if you are is that working ok or maybe its packed up it might still be making a noise but brushes could have worn out, or maybe when you put everything back together after you put in new pre filters something wasn't quite in place I did this once and coudnt work out why it was so slow, and one of the membranes I hadn't pushed in all the way. there a pain but it might be worth your while checking again, im talking about the 3 membranes not pre filters. might worth a try.