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.