I will have more input in a few days that will hopefully help. I have the same problem happening right now in the house we just built in May. As Carolyn, the smell originates in one part of our basement and seems to start 15 min. after we first run the water in the morning. We have city water and sewer.
I called the town Waste Water Treament office today and they are running an exterior inspection. I need to have a plumber to inspect the inside. Once I learn the issue I will be sure to drop off more info.