Frigidaire Crown Ultra Quiet dishwasher not cleaning?

Our Frigidaire Crown Ultra Quiet dishwasher (model FDBC24BAS3) is leaving a scum on dishes and we're trying to figure out what the problem is. We've noticed that it takes a while for the water to start once the dial is turned to a certain cycle setting. During the minute or so it takes, there's a moderately loud grinding noise. Not sure if that's related. Also, the drain is always full of water. Not enough to cover the bottom of the dishwasher, just enough to fill the drain cup. Perhaps that's normal? Anyway, any insight would be appreciated.