It seems as if it's coming from the structure, not the sink like others suggested, as you say it comes from the vents and don't mention having this smell only when the water is turned on. The only thing I can think of therefore, and which others have suggested, is Chinese drywall, which smells like sulfur. The Environmental Protection Agency confirms there is a problem with drywall, but says the breadth and severity of that problem are unknown. “It is the drywall, and from what I gather it is causing a problem with copper and, specifically, air conditioning units,” said Dawn Harris-Young, spokeswoman from the EPA’s Region 4 in Atlanta. After new home buyers in Fort Myers Florida complained about the sulfur odors, the builder had the Chinese drywall tested and determined that it was the problem. The builder replaced the drywall in one house and used ozone, fogging and barrier paint in the other. Florida health department officials have received over 50 complaints from around the state about these problems. It can make the coils and copper in your a/c turn black, so some people say this can be a hint of the problem, or if your a/c coils need constant replacement. Good luck, I hope for your sake it isn't this problem, as it is hard to get the builder to cover your costs of having to basically redo your home due to the drywall they chose to use. This is even harder to accomplish when builders go bankrupt or take off, something that's becoming very common nowadays.