If you notice an odor emanating from your drains, sulfur is probably to blame. To be more precise, microorganisms in the pipes are what produce hydrogen sulfide gas. While it's important to point out that this isn't the sole cause of odorous drains, it is highly prevalent. It would be best if you cleaned the pipes to solve the issue. You may pour half a cup of bleach or, alternately, half a cup of baking soda and half a cup of vinegar down the drain. Remember that using the latter of the two will result in a little chemical reaction that will allow the foam to rise back up the pipe; it is thus advisable to block the drain after pouring the vinegar to press the foam through the pipe. After using either technique, pour warm to hot water down the drain. You should contact your neighborhood plumber if this does not work.