Showerthought: it took me 3 weeks to figure out why my kitchen sink smelled, and it was one tiny thing
Started noticing a weird smell coming from under the sink about a month ago and figured it was the garbage disposal, so I ran lemon peels and ice through it twice, then dumped baking soda and vinegar down the drain, then bought a $9 drain snake. Nothing worked, and the smell got worse every time I ran the dishwasher. Turns out the little rubber flap on the dishwasher drain hose had gotten flipped backward during a clog I cleared back in June, so dirty water was sitting in the hose loop for weeks. Flipped it back with a butter knife and the smell was gone in a day, but I still can't believe I bought a snake for nothing. Has anybody else had a smell that turned out to be some dumb little flap or seal instead of the actual drain?
Had the same thing happen with my washing machine drain hose a couple years back. Water kept backing up and the whole laundry room smelled like a swamp and I was ready to call a plumber. Turns out the hose had a little kink in it and the check valve thing was stuck open so nasty water just sat there. What fixed it for me was pulling the hose out, straightening it, and running a bucket of hot water with dish soap through it to flush the gunk out. I also started checking that flap valve every few months now because once it flips or gets gummed up it's the same problem all over again. The real lesson is to check the cheap stuff first before buying tools or dumping chemicals down there. Those drain snakes are handy but they won't do a thing if the problem is a flipped seal letting water sit.