I have noticed that when my washer cycle is finished and I open the lid while its finishing the spin cycle I can hear water under the steel drum as if it is not drained all the way. This is driving me nuts and even though I leave the door open when not in use it still gets a mildewy smell. I have physically drained the hose out once before and the smell was gross. I don't know if this is a pump issue or something else.
It is supposed to have water underneath the drum; it is only an issue if it backs up into the drum. That water gets changed every time you run the machine. It might be the rubber lining that smells and not the water below the drum. You can try cleaning the tub with Oxyclean and leaving the door propped open to allow ventilation after you use the washing machine. Often the machines have a magnet on the latch that lets you keep the washing machine propped open but not fully open.