Home built in 1995. We changed faucets and countertop 11 months ago. now we get a sewer smell from one of the sinks when we turn the water on. It does not do it every time. We have cleaned the pipe via rooter. City water dept. said the trap is burping. only one sink does it and the other is only 4 feet away. any ideas? trap etc all the same as before. We also cleaned the overflow drain.