I have a manufactured house by patriot and in the master bath, I rarely use the sink. When I do use the sink, the water, not the drain smells like sewage, but only the cold water. I use the bathtub in this bathroom but I don't have any problem with the water there. What could be causing this problem?

Does it taste oily?

The water smells like poop so I don't drink it. It doesn't appear to have an oily appearance.