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