bad smell

Alright this is a weird problem, but everytime I run the water in the kitchen long enough I get a horrible smell, probably sewer gas, permeating my apartment. It also occasionally comes up even when I don't run the water, but it comes every time if I run it for a few minutes. The bizarre thing is the smell comes out of the air conditioning ducts; the smell is much stronger when the AC is running.
It's been suggested that it might be a crack in the vent pipe, which I guess makes sense. I've been on the roof and the pipe doesn't seem clogged (at least where I can see it), and the trap in the kitchen sink is newly installed.
My questions: 1. Is there any way to pinpoint where exactly a leak is (if one exists)? The AC unit isn't especially complex, and the ducts aren't that extensive; I can see almost all of the duct length by standing on a chair and shining a flashlight into the vents. I don't see any possible leak though. The air conditioning unit on the roof for my apt isn't close enough to the vent pipe to suck in any smells either.
2. Should I call a plumber or an AC repair person? Presumably a plumber would be needed to actually repair any pipes, but I would think an AC person would be better able to find the leak.
3. Is there a way to search for leaks without taking apart walls?
4. Is there any possible other explanation for the smell?
If at all possible, please post a response here, my email account is inaccessible at the moment.
Thanks for any help anyone can provide.
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http://groups.google.com/group/alt.home.repair/search?group=alt.home.repair&q=smell+water&qt_g=1&searchnow=Search+this+group
snipped-for-privacy@hotmail.com wrote:

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Sounds strange. One thing to check out is the drain pan for the AC. I have to wonder if it is draining into the vent line and something is broken or clogged.
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snipped-for-privacy@hotmail.com wrote:

Hello: when water runs, it is possible to evacuate other traps in the house that are not vented properly. If an ac return duct is near a trap that's been evacuated this could cause smells to enter ac system. next time you get smell, check the other traps.
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Just wanted to let you know that we're experiencing a similar problem- a weird sewage smell coming out of the AC. How bizarre! It's baffling to us and we're running out of ideas.
We started by having the ducts cleaned, then we changed the filters a few times. When we asked our HOA president, he made it clear he thought we just had digestive issues. We had an AC expert out to check out the system. He confirmed that it's a weird problem and said that he'd never come across it before and wished us luck. We had the plumber out who charged a bunch of money and we're still having the problem! It's getting a little embarrassing not to mention costly.
Our symptoms are 1) strange smells coming from AC/heat ranging from sulfer to sewage to garbage to stale smoke to a fridge that needs cleaning 2) the smells are not always present and vary in strength 3) we haven't noticed definitively that anything else we're doing in the house affects the smell but we have suspicions that when we run the washer/dryer it's worse (the vent and the w/d are located in the same small space 4) our neighbors upstairs recently remodelled 5) we've just moved in about 3 months ago. according to the neighbors, the people who lived here before us never complained about any strange smells so we're assuming this is a new problem. 6) our duct work runs through the upstair's neighbor's condo.
Reading the responses to your email is definitely helpful. I thought you might like to know that you're not the only one out there. If you come across a solution, please let us know. We're getting a bit desperate. Please, somebody have an answer!
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I pass this on hoping it may help someone. Several years ago in my son's bedroom came a smell likened to dead fish. It came and went. We had all the neighbors and their dogs sniffing for the cause, unsuccessfully! I spent untold hours and finally decided to open the walls and look for a dead rodent. Before it got that far somehow I looked at the ceiling fan lamps/holders. The insulation in the lamp holders was apparently treated with an organic oil, fish oil? Made in Taiwan.. When the lamps were illuminated the odor became very offensive. Discarded the cheap holders and installed ceramic type. No more odors. Best, Seamus J. Wilson

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