It may have to do with buildup of residue (soap scum, scale, lime, etc.) in the recirculation system. I contacted LG about this on numerous occasions and they advised running white vinegar or a commercial cleaner through the system. I tried both vinegar and commercial cleaners numerous times, with no positive results. Then, quite by accident, we switched dishwashing detergent from Cascade liquid to '''Cascade Platinum pods with Dawn'''. After about a month using the pods, the dishwasher is as quiet as it was when it was new, with absolutely no whistling.