Questionable funk from the air vents

[SilverBack]

Okay, so my deal is that my vents have been shooting this stale, 3-week-old tortilla-chip smell everytime I start the car with the A/C on. It's only for a few seconds, then it either goes away or I get used to it. This is has been going on since the lovely new-car smell faded away at 4,000 miles. Since then, I've been dealing with this everyday and it's really starting to get on my nerves. I only have about 7,700 miles on it, so I'd say it's way too early for my car to stink itself up. Toyota dealer found nothing wrong with the A/C, filter and fan, and even sprayed air-freshener for free, which fades away in minutes. So what's really wrong with it? How do it fix it for good? Thanks.