The overflow is a little hole at the top of the sink that connects to the bottom of the sink. It is also susceptible to slime buildup. If you are on a city sewer system, you can try to pour a 50-50 mix of chlorine bleach and water into this hole. A turkey baster works well for this purpose. Never mix chlorine bleach with ammonia or other cleaning liquids.
Toxic gases can form.
If you are on a septic system, do not use chlorine bleach. It can harm the bacteria in your septic tank. If after all of this the odor persists, the problem may be a serious plumbing problem. The toilet seal may be broken. A hidden vent pipe may have cracked.