It smells like really musty water, but the water itself does not smell or taste bad at all. Only happens when I run the water, so it doesn't make the room smell, just when you fill something up or wash your hands.
I tried drain-o and it didn't do anything and there's GREAT flow so it's not clogged at all anyway.
Only thing that has worked (temporarily) is pouring pretty much everything I could think of into it. Lemon juice, draino, bleach, etc.
I'll echo the comment and say that you should make sure you have a proper P-trap installed under the sink. This trap holds water and provides a seal against sewer gases getting up into the bathroom. Without one, gases will leak in constantly, and will be displaced by water down the drain which can force the gases up into the bathroom even if normally it's not detectable.
I'll also ask if there is a proper vent for this sink. Even with a P-trap, if there's nowhere for back-flowing gases to go, they'll bubble up past the trap.
One more thing it could be is the overflow drain. Depending on the design of sink, the overflow can hold a small amount of water at the bottom where it tees in to the main drain, which can become stagnant. Run water down the drain and you'll force some air up the overflow (to make way for the water coming down), which will have that stagnant smell. To diagnose this, plug the sink and begin filling it; you shouldn't get any musty smell at first because there's no air movement. Once the water level hits the overflow drain, you will start smelling the musty smell for a while because the water is displacing the gas, which wants to rise above the water and so will move up into the bathroom. If this is the problem, you can ameliorate it with some foaming pipe snake; pour it down the overflow drain and it will clean out any caked-on gunk which contributes to the smell, and which may be trapping the water. The real fix is to make sure there's no "damming" effect of construction defects at the bottom of the overflow drain (a lip of porcelain, issues where the overflow meets the metal drain downpipe, etc).
Does it smell when you turn on the tap and catch the water in a bowl (so it doesn't go down the drain)?
If so, it's something in the faucet. Take off the aerator cap and look for gunk inside, and/or look in the barrel of the faucet if you can to clean it out. You can also consider replacing the faucet. (I am assuming that since this only affects the bathroom, it is not a water quality issue or bad house pipes)
If not, it's something in the basin, drain, trap, or overflow drain. KeithS has some good answers, and I also recommend the following:
From what I can ascertain, it's when the water is running. It's not that common, but we have had water that smelled like rotten eggs. The cause was sulfur in the water.
If you're on municipal water, check with your utility. If you're on a well, have the water tested by a lab. They'll tell you what the story is.
We tried all that stuff, including flushing out the vent pipes with a hose to check for blockages; nothing worked. I decided to crawl under the house to see if there was a sewer leak - there was a dead cat under the house! If all else fails, check your crawlspace. On the plus side, we now have what are probably the cleanest pipes and septic tank in town. :-)
For years, I tried to find the source of my stinky sinks. Not just the kitchen sink, but my bathroom sinks also had a 'sewer odor'. I cleaned sinks, pipes, garbage disposal, etc. I cleaned, poured, disassembled and checked traps, cleaned traps, you name it; I did it.
FINALLY, someone suggested that I clean my dishwasher. I never would have thought of this. Isn't the dishwasher inherently "clean"? I just used a commercially available dishwasher cleaner (Cascade, Finish, etc. all sell them). This is not a dishwasher detergent but rather a Dishwasher Cleaner. Once a month I run one of these babies through my empty dishwasher on the very hot Sanitize setting and VOILA, no more stinky sinks! Do yourself a favor and at least give this a try. You will be amazed!
If the smell starts only after you turn on the water, it is probably coming from the overflow. Take the stopper out and block the drain with a plastic baggy with a wet rag in it. This has to block the overflow drain where it meets the regular drain. Now pour the foaming snake, or other drain cleaner into the overflow. Let stand then reconnect the plumbing and flush with hot water.
One of my children stuffed a bunch of peanuts down the overflow drain of our bathroom sink. I was able to get it out with a wet dry vac, some wire to poke the stuff through and a lot of patience.
Instead of using expensive garbage disposal deodorizers I just mix one pack of Lemonade flavor Kool-Aid (10 cents a pack) with a small amount of water. Then pour that into the sink and let it sit for a while. I also use a long handled brush to clean the bottom of the rubber guard in the sink because junk gets under there.
