There may be a layer of bacteria lining your bathroom sink over-flow drain, which, cause a FOUL smell.
If you notice a foul odor coming from the overflow pipe in your sink, the pipe is probably harboring a layer of gunk and a colony of bacteria. Even though you can’t access this pipe as easily as you can access the drain itself, you can still clean it fairly quickly. You might be able to do the job using only household items, but if vinegar and baking soda don’t do the trick, try oxygen bleach as a less-toxic alternative to commercial bleach.
Combine 1/2 cup of baking soda and 1/4 cup of table salt to make a natural scouring mixture. Pour it down the sink.
Pour 1 cup of warm distilled white vinegar down the sink. Wait while it foams, and let it work for 15 minutes. Notice whether any of the mixture emerges through the overflow pipe; it may or may not, depending on the force of the foaming action in the drain. If it doesn’t, you are more likely to need the oxygen bleach later.
Flush the drain with hot water for 30 seconds. Fill a turkey baster with hot water, and vigorously squirt the water into the overflow pipe to rinse that portion of the drain system as well.
Check whether the overflow pipe still smells. If so, the cleaning mixture didn’t foam far enough up the pipe to reach the problem. Open a window for ventilation, and fill the turkey baster with oxygen bleach.
Insert the tip of the turkey baster into the overflow pipe. Forcefully squirt the oxygen bleach into the pipe. Repeat this three to four times to cover as much of the inside of the pipe as possible. Let the oxygen bleach sit for 15 minutes, or the length of time specified by the manufacturer.
Rinse the pipe thoroughly by squirting in hot water with the turkey baster. Dip a cloth in some distilled white vinegar and wipe the inside of the overflow pipe to kill any bacteria living in the mouth of the opening. Allow the vinegar to evaporate.