Water smell from one sink

I have a sulfurous smell coming from my master bathroom sink. Here are the clues:

1) The master bath is probably the highest plumbing location in the house.
2) This is a double sink. Only the left sink smells foul; the right one smells fine. The shower smells fine. The toilet smells fine (when not in use )
3) The foul odor from the left sink is intermittent; I smell it several nights a week when brushing my teeth. But in the morning it's fine. Sometimes, it'll disappear for weeks, only to eventually return.
4) I poured bleach down the sink once, and it stopped the problem for a couple weeks, but it came back.
5) I have a septic system. My house is 3 years old, and the septic is also that old. It's rated for 5 bathrooms, and we have 3 1/2.
6) I checked the trap for buildup, but it's clean as a whistle.
7) I don't notice any patterns with the weather; it does not smell more or less during any one season, or depending on the rainfall.

Any ideas as to what's causing this? Better yet - any cures?

3 things come to mind.

#1 Check if the pitch from the trap to the wall has fall towards the wall. If not correct this and while you are doing that remove the trap and arm and clean it out of any build up inside.

#2 Does the drain drain slowly. If so snake the drain out.

#3 Your pop up may need to be replaced due to crud build up in parts of the assembly that can't be reached / cleaned. This is not a common thing but I have seen it.

Thanks, Cass.
The trap pitches properly towards the wall.
The sink drains lickety split; no slow-draining probs there.
I checked the pop up when I checked the trap. Both were clean.

Any other thoughts?

Just guessing that one sink is used much more than the other and as Cass says, take the pop-up out and clean it and run a bottle brush of some kind down into the drain. You may even want to mix a solution of water and clorox or ammonia and overflow it to wash out that area too.

The parts of the pop up I was referring to can't be seen unless you disassemble the pop up. If it has any any age to it you may just want to replace it.

Ok, I'll give the pop up a good scouring and bleach the trap/drain nice and good. Let's see what happens. Thanks for the advice!

Run some bleach down the overflow tube...sometimes the overflow starts growing some nasties...

Bingo!

Thanks for all your help! It was the overflow tube! It seems that the builder plugged the overflow holes in the drain with plumbers putty when installed. I can't understand why.

Anyway, when I removed the putty, and ran a little water down the overflow, I smelled the bad odor.

I believe that without drainage, something has been growing in the tube. When it's dry, there's no problem, but when I occasionally splash water in there, it gets moist enough to emit the odor. Perhaps it doesn't get moist enough to smell until the evening, after a day's use.

Regardless, you can bet I'm gonna give that bad boy a bleach enema today.

THANKS!