Our bathroom sink smells like sewer when we run water down it. I know the trap is full because we use this sink every day. We have to keep the drain closed and run a sink full of water before we can open the drain and use the sink. What is causing this problem? Could our vent be plugged? It seems to get worse when we flush the toilet in that bathroom. Flushing anywhere else in the house doesn't seem to cause any problems.

Do you know if you have a P trap or an S trap? Does the trap go into the wall or the floor? A plugged vent would most likely cause chugging when you drain the sink, like turning a full bottle of pop upside down and letting it drain. You may also have a batch of hair, grease and other stuff fermenting in your drain. Try a gallon of 50/50 mix of bleach and water down the drain and let it sit overnight. Come back with your type of trap please.