Plumbing Q: Stinky Sink drain .. any ideas?

About two years ago, I had my 2nd story bathroom remodeled. For months, it was completely fine, but recently, there's a bad smell from the bathroom sink. It drains well .. But to get rid of the smell, I did try a draino type liquid. Didn't fix the problem. This has been going on for months now.
It smells like rotten eggs .. maybe sewer gas. As I said, the plubming under the sink is updated, up to code, and has a trap there, which would make me think it's not sewer gas. -- I've opened up the trap there, and it's clean as a whistle.
Also, generally, you smell it more when the water is on.
Could it be something like my vent pipe to the roof is clogged, forcing gas through the trap? -- No other drain in the house has this problem, btw.
Any ideas? Thanks in advance.
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<snip>.

It's possible to have a blockage in the roof vent pipe. To clear your stacks just feed a running hose down the pipe until it stops. I've done it - obviously a no brainer give that little tidbit.

This part is intriguing to me because I had a similar situation. You didn't mention your water source so if it's chlorinated public water this won't help. I had bacterial growth in the water heating tank which required a shock treatment to kill off the bloom, but this was at a cabin with well water source which has a low bacteria count. Unfortunately for me, the tank became a incubator when used occassionally and turned off when not in use. It took 5 or 6 months for this to happen.

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Please let us know what you mean by a "drain issue". I ask because I have a similar problem: I get this musty smell only in the spring and summer, the drain is fine in the winter. If I put down the drain stopper, it is better (although not 100% because I can still smell it a bit from the small opening at the top of the sink).
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