This is a weird one, and have come across multiple handymen and none of them can pinpoint the problem when I describe it. Though they have never been in the room when it happens.
Here is the scenario...
We leave the central A/C on all day because we have big dogs in the CA heat, so when we get home, the A/C is running. When we turn on the water faucet in the kitchen sink, immediately one of the bathrooms emits a sewer odor that lasts for about 10 minutes. This happens consistently so much that the first thing when I get home is to open the bathroom window before my wife starts using the sink.
I have placed my face in the toilet, bathroom sink, shower drain, and A/C vent in the bathroom and cannot pinpoint where the smell is coming from. The bathroom in question has a wall that faces the front of the house and that part of the house is where the main water supply line is. When the scenario with the smell is happening in the bathroom, if I go outside my house into that part of the front of the house, I can also smell the bad odor, but not as strongly as in the bathroom.
I once had a plumber here to see if I could create the smell. It was about 9am and I turned on the A/C for about 20 minutes then turned on the kitchen faucet, and OF COURSE it didn't smell, so he couldn't help pinpoint the situation. I guess a prerequisite is that the A/C be on for hours or all day.
In conclusion, the smell doesn't come in any other scenario. It's just the combination of the A/C and kitchen sink (no other sink in the house causes it). And the smell is only that bathroom. I have another bathroom and no problem there. I've had the HVAC unit for 3 years and it's really been happening for a year now.
Ask me any questions you may have and thank you in advance for reading this long story and for any input you may have.