Is the smell coming out of the sink drain? If so, it sounds like perhaps you don't have a p-trap under it, or it is either installed improperly, or the vent pipe is clogged. The water trapped in the bottom curve of the p-trap should block any sewer smell from coming up. If the smell is coming from somewhere else, it could be a leak in the vent pipe behind the wall. A leak will let sewer gas out directly, but a clog could cause a siphon situation that sucks all the water out of the p-trap, leaving no slug behind to block the gas. Another possibility is that the water coming out of the faucet is the culprit, meaning that the pipe to it is contaminated with something, but that is not likely. Check the p-trap and the vent line. You can run a snake down the vent from the roof to see if it is clogged.

I had a smelly bathroom sink once. It would smell for a few seconds after I started running water, then dissipate. I suspected trapped gas in the pipes or something. When I had a plumber over for an unrelated problem he said that the smell might come from the space between the overflow opening and the drain. Stuff gets caught in there over time and is just about impossible to clean. When you turn on the water, the gas that sat in that space gets forced out of the overflow hole. Something like that. I dribbled some disinfectant cleaner into the hole and a bottle cleaner to wipe around a bit, then rinsed. That seemed to have helped.

I've had a bad smell coming from my sink. Last fall I pured Liquid Plumber in, let it set and flushed it down. I poured vinegar (straight) in, let it set and flushed it down. I used bleach and flushed it down. Each time, after a day the smell came back. We left for 3 months this winter and when we came back there was no smell. Now,after a week it started again; I poured Simple Green in the sink-straight so it would set in the goose neck pipe underneath; then flushed with HOT water in about an hour. Smell went away, then 2 days later it is back again. Now...I am thinking that because it's an earthy/moldy smell,, could it be a broken pipe underneath and the insulation is wet and rotten ... could that be it? We also remodeled that bathroom 4 years ago and had a new sink put in, but not new plumbing. HELP PLEASE! We use Mentadent toothpaste, but rinse it well with warm water afterwards. Our other bathroom has never been remodeled and hubby uses it every day and it doesn't smell at all.

You were all so helpful with your comments, I wanted to let you know the outcome of my smelly bathroom. . . It took three different plumbers and a camera to finally determine that we were getting a smell from the shower drain because there was not a properly installed p-trap for the shower. This was particularly surprising since the shower was done by a well known and reputable contractor who came highly recommended and the bathroom was done with permits so the city inspector signed off on the bathroom. I sent the original contractor (from 3.5 years ago) the video from the camera guy and they came out, opened up the ceiling on the first floor and inserted the p-trap. And now the smell is gone.