I have a old house that was buit in 1950. When you run the water in the kitchen and bathroom a foul oder comes out of the sink drains. The kitchen and the rest of the plumbing in the house drains fine, I looked down the roof vent and I can see standing water at the bottom of the vent. Both sinks have p-traps. Any ideas?

You may have a clog that is sending bubbles up the drain.