Persistent odors from a drain usually mean that organic material is stuck deep in the pipe, creating bacteria growth. I once had this issue, and no amount of boiling water or vinegar fixed it. Eventually, I hired a service to clean it properly — I found them here:
https://bens.plumbing/services/drain-cleaning-seattle/. Their equipment flushed the buildup thoroughly, and within minutes the smell was gone. What’s more, they explained how grease and food residue stick to pipe walls, causing odors even without clogs. The cleaning process not only removed the smell but also improved drainage speed. Since then, I’ve been more careful about what goes down the sink. I also occasionally use enzyme-based cleaners as a preventative measure. If your drain smells despite surface cleaning, this deeper approach might be the real fix.