I run the washing machine every Thursday and Sunday. During the first rinse cycle (when the water drains) of the first load I run on each of these days, an unbelievably horrible sewer gas smell comes into the house. This is the only time we notice it. We were afraid the vent might be clogged, but if so, wouldn't it stink when we ran the faucet, took a shower, or flushed the toilet? And it only happens on the first rinse cycle of the first load.