Why Does My Washing Machine Smell/Stink?

Likely a combination of things. I assume it is warm down there, so any organizms can grow easily. Research some disinfectants and use them. Bleach might be another option. At the worst, get under there and put a Tee at the lowest point where the outlet hose connects. Bring a small hose off that with some sort of cap/ valve setup so you can fully drain the pump/ hose. There is ALWAYS some water trapped in the workings