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Our 20+ year old washing machine has started to partially fill on it's own when not in use with stinky brown water. Is this a washing machine problem or a plumbing/sewer problem?
......... How is the old girl drained?
Probably a leaking solenoid valve and a rusting tub or pipes.
I would agree with Jack because the drain connection is not air/water tight, so if it were backing up you'd have a mess all over the floor, rather than just in the machine. In addition to the leaking solenoid and rusty tub, it may not be fully purging the waste water either. Consider yourself lucky to have gotten 20 years out of it and invest in a new machine.