Smelly sink

I have 2 bathrooms in my house -- one on the main floor and one on the second floor. I haven't noticed any odors on the second floor, but I can smell something coming from my main floor sink. It only happens when I turn the water on and goes away when the water is shut off. I've also noticed that when using the toilet, the smell seems to appear. Whether it's coming from the toilet or from the sink at that point is hard to tell.
I've used Septo-bac a few times but this doesn't do much, if anything. I've heard that this can happen if the water in the trap dries up, but the sink is used quite often and I can see water when I look down the drain.
I've heard bacteria can build up in the overflow drain, but this is an old sink and doesn't have an overflow.
Other thing I've noticed is this drain is somewhat slow moving and tends to clog on a regular basis.
What should I look at next?
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KD wrote:

Only smells while water is *running*? If you have well water, suspect that rather than the drain. Jim
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I'm on town water and this is the only sink that smells. Seems like the smell comes when the water in the trap gets disturbed.

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KD wrote:

Possible it's mildew growth in the drain tailpiece just above the trap. Swab it out with a brush soaked in bleach.
For the toilet, add some bleach to the water in the tank which will then flush the openings in the bowl. Jim

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Two things come to mind right off, one far less problematic than the other. What the smell smells like will pretty much tell you which one you might have:
1) If it's just gunk-stinky that you kind of have to work at catching a whiff of, you might gave a LOT of years-old slimy yuck (hair, toothpaste, hocker spit, fingernail clippings, puke, etc. and whatnot) built up on the inside of your pipe between the basin and the U-bend trap. Take a wrench, unscrew the U-bend portion (make sure you put a bucket under there to catch the water that's trapped once you unscrew it), and clean out the U and the straight section of pipe leading from the basin. On a lot of sinks, you can even unscrew the straight section.
Or, 2) If it smells like death rising up out of the drain, it's possible/probable that you're smelling sewer gas because your sink and/or bathtub (they're often on the same pipe) is not properly piped into the main sewage drain down in the basement. Go into your basement and find the big honkin' black cast iron or white PVC plastic line about the diameter of a small tree attached to your toilet on floor above you. Nearby, you'll also see (or *should* see, at any rate) a U-shaped pipe/trap coming down from where the sink and bathtub next to the upstairs toilet are located. There's a pipe from the sink/tub running into the U joint, and another pipe running from it. The pipe running *into* the U should be set several inches *higher* than the pipe running out of the U and into the big honkin' line. Betcha dollars to donuts that some DIY plumber somewhere down the line put the pipe after the U either on the same level or lower than the pipe running into the U. If this is the case, a reliable plumber can fix this, and it's a pretty easy fix.
AJS
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