We are having a weird problem in our bathroom. When we haven't used it for a couple of hours, then flush the toilet a weird smell comes up out of the sink. It's not a sewer gas yucky smell. It's more like car exhaust. Really weird! We live in Washington State, it's pretty mild this summer. 65-80 degrees or so. The house was built in the 40's and completely remodeled before we bought it 3 years ago. Haven't had any problems until now. Any ideas??