Why would your house smell like sewer whenever you run the washer or dishwasher?

I suspect that you have a "p" trap in either a floor drain, or possible the laundry tub or a sink somewhere in the house. If you look under the kitchen sink, or a lavatory in the bathroom you can see a p-trap, it's shaped sort of like the curved part of the letter p. The purpose of the p-trap is to trap, or prevent, sewer gasses in the sewer lines from backing up into the house. By design, p-traps catch and hold some water, which blocks the pipe and stops the gas from coming up through the drain. IF you have a floor drain, or a lavatory drain, either of which don't get used enough to keep the water in the trap from evaporating away, leaving the drain pipe open, then when a lot of water is poured or flushed, pumped out of a washer, then that flowing water will push sewer gasses back up through the drain with the dry p-trap. What you are smelling is sewer gas.