One AC vent gives out smell

In this 18-year-old house, the A/C vent at the entrance to the master bathroom sometimes gives out strong bad smell. We can't find a pattern, whether it's related to weather, or we just used the toilet or shower, etc. The smell is sometimes like urine, occasionally like sewage. It happens maybe one out of ten days. I can't see anything by shining a flashlight into the hose, and can't visually see anything unusual when I go to the attic. No other A/C vent in the whole house has any smell at all. How do I go about troubleshooting? Thanks for advice.
Yong Huang
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You may have a critter or two living in the duct. Remove the entire flexible section and bring it outside. Just get new flex to install or better yet get solid steel duct to eliminate the problem from happening again. Insulate the steel with duct wrap.
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Thanks, John. How did they get in? Drilled a hole on the duct? So I'm actually leaking cool air into attic?
Before I get a contractor for the duct work you suggested, do you think I can remove the A/C register, put some food on the ladder, and place the ladder under the A/C vent to get them out? It's hard to imagine they can live in a 100 F environment.
Yong Huang
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wrote:

If it is flexible duct almost any rodent can chew into it. I don't recommend enticing vermin into your house. Call an exterminator to have a look.
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It could be mold
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Yong Huang wrote:

Have you inspected the attic floor for droppings? Might have bats
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On Tue, 8 Jul 2008 15:19:36 -0700 (PDT), Yong Huang

I had this. Only in one vent, when the AC was on, which I only use about one month out of the year. When the heat was on months later, I smelled nothing, but I did smell it again when the AC was on the following year.. It smelled like a damp dirty basement, maybe a weak sewage smell, but never urine. (My basement where the furance/AC is is very dry and didn't have a smell.)
Maybe a mouse got in there and died, but I don't think I had mice that year. :) If it was a mouse, in the open, like the basement floor, it woudl dry up entirely in a month or three. I don't know how this smell lasted to the following year, but I think that was the end of it.
The ducts to this register are between the first and second floor, and in the wall. I hoped it would go away, and it did after 13 months.
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