Problem with smelly sinks?

Hello,

We have an odor in our master bath that's like old gym clothes. It seems to come from the two sinks. When the bathroom is closed up, it takes a few hours for the odor to build up, and it gets worse when the weather is cool and damp.

I've flushed the overflow and the drain with chlorine bleach, baking soda and vinegar, and toilet bowl cleaners.These help for a day or so, then the odor returns. I have tried cleaning out the p-traps, which appear to be functioning properly. I've tested the plumbing vents for leaks, none found. I've checked in the crawlspace for leaks and odors there, and in the attic, but found nothing unusual. This is part of a new addition built in 2013. People have suggested testing the water, but no other part of the house has this problem.

Any suggestions for what else I can try? The odor is driving us nuts.

Thanks,

Greg