A few months ago I replaced a leaking faucet on my bathroom sink (located in the master bedroom). As part of the job, I also replaced the drain tailpiece, and P-trap. A few days later, I noticed that when running the water (usually slowly), a bit of sewer gas would come up from the drain. It happens nearly all the time, but clears up after a few seconds. What's odd is that there is no smell at any other time; i.e. if no one turns the water on, there is no smell. It's only during the first 5-10 seconds after turning on the water.....then it goes away. I replaced the P-trap with another new one, but the problem still remains. Before I changed out the faucet, we had never had any problems like this. And we have lived in the house for 22 years. About a month later, I remodeled the main bath which is on the other side of the wall from the aforementioned problem sink. As part of the job, I replaced both faucets in a double sink along with the respective tailpieces and P-traps and have had no problems. I would imagine that these faucets share the same main soil stack with the problem faucet, but probably have separate vent lines. Any ideas?