Strange Smell from Vents

I have a 2002 Ford Escape. Every now and then, I get a strange vinegry/acidy smell coming from the A/C vents. It occurs when I use the fan or the A/C. It
last for a couple of days, goes away for a few days, and then the cycle begins again.
Any idea what could be causing this and what I can do to get rid of it? Thanks.
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Since I'm no mechanic...where is the drainn pipe on the 2002 Escape? I know I see water dripping from underneath the engine after I've been in the car awhile and running the A/C. I assumed this was the drain pipe.
Could the smell also be coming from the windshield washer fluid or glass cleaner I use (Rain x)?
Thaniks.

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It's on the passenger side right behind the alternator. It's just a 1/2" tube sticking out about 3/4" from the firewall. Suprisingly, it doesn't point down at a right angle like all the other cars I've had.
RainX doesn't have a vinegary smell; more like alcohol. And unless you've added vinegar to the washer fluid, I wouldn't think that would be it either. Smell it and see. Check around your battery and make sure no acid has boiled over- that might give you the smell. Tom F.

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Someone I know who bought a car from the same dealer (Hertz) said he also had the smell and was told it was from the cleaning stuff they use when detailing the car. He said the smell eventually went away.
I'll check around the drain pipe and the battery, anyway. Although I had a mechanic look at the car a week ago and he found nothing wrong.

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I think I agree with the mildew idea. We have a 1990 honda accord, and when you turn on the AC, for probably 10 minutes or so (gradually decreasing) there is an absoloutly horrible smell. My dad read somewhere that sometimes water doesn't drain properly, mixed with whatever debris picked up from the road (leaves, bugs, dirt) and gets mildewy. He read (in the same article) that parking a certain direction on hills could have some to do with it. Just a few days ago, we started parking it facing down hill in the drive way, to see if that actually has an effect the draining of condesation and therefore mildew forming.
Now when you say vinegry/acidy I'im not sure if that's the type of smell you smell. Cant you smell that smell? ...anyway. you might see if that helps.
Nic

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Make sure you dont have a cat that likes to sleep underneath your vehicle. They have the nastiest urine smell of most animals and it WILL stick to your system if they do.

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