If it's coming from the sink only, and not the tub or any of the other sinks in the house, then I'd suspect you have something rotting in the overflow channel of the sink. This is where hair, soap, and stuff accumulate, all of which are organic and will rot. You can clean this out by:
1. remove the trap under the sink.
2. Stuff a rag into a plastic bag and stuff that into the drain pipe under the sink. (Support the plastic bag from below with a piece of wood inside a pail that prevents the plastic bag from coming out of the drain pipe.)
3. Now fill the sink with water so that the water fills the overflow channel.
4. Now plunger the sink. The water surge will go up the overflow channel and a bunch of crap will end up coming out of it.
5. Remove the wood support, pull the bag out and drain the water into the pail.
6. Repeat as necessary until the overflow channel is clear.
If there's been any construction involving connecting or disconnecting water mains on your street lately, it's very possible that there's a infestation sulphur reducing bacteria in the water supply piping. Normally the chlorine in city water will prevent sulphur reducing bacteria from thriving in water supply piping, but it could be that the chlorine contend of the water in your area isn't high enough to do the job.
A. O. Smith Water Heaters
hover over "LIterature" and click on "Technical Bulletins"
Download Bulletin #22 entitled "Smelly Water".